Channel state information sending method, channel state information receiving method, and device

ABSTRACT

Channel state information (CSI) sending methods, apparatuses, and systems are disclosed. In an implementation, a method comprises: receiving, by a network device from a terminal device, channel state information (CSI) comprising a rank indicator (RI), indication information, and a second precoding matrix indicator (PMI2); determining, by the network device, the RI and the indication information from the CSI; determining, by the network device, the PMI2 based on the RI and the indication information; and determining, by the network device, a precoding matrix W based on the RI and the PMI2, wherein W satisfies W=W1×W2, W is an Nt by L matrix, W1 is an Nt by 2I matrix, W2 is a 2I by L matrix, L is a rank indicated by the RI, Nt is greater than or equal to L, and I is an integer greater than or equal to 1.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/696,334, filed on Nov. 26, 2019, which is a continuation of U.S.patent application Ser. No. 16/279,329, filed on Feb. 19, 2019, now U.S.Pat. No. 10,511,367, which is a continuation of U.S. patent applicationSer. No. 16/146,599, filed on Sep. 28, 2018, now U.S. Pat. No.10,270,505, which is a continuation of International Application No.PCT/CN2018/083967, filed on Apr. 20, 2018, which claims priority toChinese Patent Application No. 201710459616.9, filed on Jun. 16, 2017.All of the aforementioned patent applications are hereby incorporated byreference in their entireties.

TECHNICAL FIELD

This application relates to the field of wireless communicationstechnologies, and in particular, to a channel state information (CSI)sending method, a CSI receiving method, and a device.

BACKGROUND

Currently, multiple-input multiple-output (MIMO) technologies are widelyapplied to communications systems, such as a long term evolution (LTE)system. In the MIMO technologies, a transmit end and a receive end eachuse a plurality of transmit antennas and receive antennas, so thatsignals are sent and received by using the plurality of antennas of thetransmit end and the receive end. The MIMO technologies can improvecommunication quality and a system channel capacity.

In a MIMO system, a precoding technology may be used to improve signaltransmission quality and a signal transmission rate. A network devicemay estimate a precoding matrix for a downlink channel based on CSI fedback by a terminal device, and then the network device uses theprecoding matrix to perform downlink transmission with the terminaldevice. For a high precision codebook-based precoding matrix defined inthe LTE standard Release 14 (Rel-14) and a new radio access technology(NR), in a prior-art technical solution used by a terminal device tofeed back CSI to a network device, the CSI fed back by the terminaldevice includes a rank indicator (RI), a precoding matrix indicator(PMI), and a channel quality indicator (CQI) of a channel matrix. ThePMI includes a PMI1 and a PMI2, the PMI1 is used to indicate allelements in a matrix W₁, the PMI2 is used to indicate all elements in amatrix W₂, and a product of W₁ and W₂ forms a precoding matrix W. Inthis solution, since W₂ is obtained based on a high precision codebookthis solution has a problem of requiring a large quantity of bits andhigh resource overhead required for feeding back the PMI2 correspondingto W₂.

As mentioned above, for the high precision codebook-based precodingmatrix, the prior-art CSI feedback technical solution has a problem ofhigh resource overhead required for CSI feedback.

SUMMARY

Embodiments of this application provide a CSI sending method, a CSIreceiving method, and a device, to reduce resource overheads requiredwhen a terminal device feeds back CSI to a network device in a scenarioof a high precision codebook-based precoding matrix.

According to a first aspect, an embodiment of this application providesa CSI sending method, including:

determining, by a terminal device, a precoding matrix W, where

W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L columns,W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a matrix with 2Irows and L columns. N_(t) is a quantity of antenna ports, L is a rankindicated by an RI, N_(t) is greater than or equal to L, and I is aninteger greater than or equal to 1; and an element at a location ini^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³is a complex number with modulus 1;

-   -   generating, by the terminal device, CSI that includes the RI,        indication information, and a second precoding matrix indicator        PMI2; where    -   the indication information is used to indicate that W₂ includes        N X_(i,l) ¹ whose values are 0, the PMI2 is used to indicate a        parameter of W₂, and the parameter of W₂ indicated by the PMI2        includes all X_(i,l) ¹ in W₂ and X_(i,l) ² and X_(i,l) ³, which        are corresponding to X_(i,l) ¹ other than the N X_(i,l) ¹ whose        values are 0, in W₂, and does not include X_(i,l) ² and X_(i,l)        ³, which are corresponding to the N X_(i,l) ¹ whose values are        0, in W₂; and    -   sending, by the terminal device, a signal including the CSI to a        network device.

X_(i,l) ¹ represents a wideband amplitude, X_(i,l) ², represents asubband amplitude, and X_(i,l) ³ represents a phase.

Alternative embodiment of this application provides a CSI sendingmethod, including:

-   -   determining, by a terminal device, a precoding matrix W, where    -   W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L        columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a        matrix with 2I rows and L columns. N_(t) is a quantity of        antenna ports, L is a rank indicated by an RI, N_(t) is greater        than or equal to L, and I is an integer greater than or equal to        1; and an element at a location in an i^(th) row and an l^(th)        column in W₂ is Y_(i,l), i is an integer greater than or equal        to 0 and less than or equal to 2I−1, l is an integer greater        than or equal to 0 and less than or equal to L−1, and Y_(i,l)        meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is a        complex number with modulus 1;    -   generating, by the terminal device, CSI that includes the RI,        indication information, and a second precoding matrix indicator        PMI2; where    -   the indication information is used to indicate that W₂ includes        N X_(i,l) ¹ whose values are 0, the PMI2 is used to indicate a        parameter of W₂, and the parameter of W₂ indicated by the PMI2        includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding to        X_(i,l) ¹ other than the N X_(i,l) ¹ whose values are 0, in W₂,        and does not include X_(i,l) ³, corresponding to the N X_(i,l) ¹        whose values are 0, in W₂; and    -   sending, by the terminal device, a signal including the CSI to a        network device.

X_(i,l) ¹ represents a wideband amplitude, and X_(i,l) ³ represents aphase.

In the foregoing methods, the CSI that is sent by the terminal device tothe network device includes the RI, the indication information, and thePMI2, so that the network device can obtain the PMI2 by using the RI andthe indication information, to determine W. In the scenario of a highprecision codebook-based precoding matrix, in the prior art, a PMI2 thatis sent by a terminal device to a network device needs to indicate aparameter of all elements of W₂. However, in the foregoing solutions,the parameter of W₂, indicated by the PMI2 that is sent by the terminaldevice to the network device, is a part of parameters of elements of W₂.Therefore, a quantity of bits required by the terminal device to sendthe PMI2 to the network device is reduced. The indication information isadded to the CSI that is sent by the terminal device to the networkdevice, so that the network device can obtain the PMI2 by using the RIand the indication information. Therefore, according to the foregoingmethods, resource overheads required by the terminal device to feed backthe CSI to the network device can be reduced in the scenario of a highprecision cod book-based precoding matrix.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂, where l is an integer greater than or equal to 0 and less        than or equal to L−1, and

${{\sum\limits_{l = 0}^{L - 1}\; N_{l}} = N};$or

-   -   the indication information includes a quantity N_(l) ⁰ of        X_(i,l) ¹ whose values are 0 in first I elements of an l^(th)        column vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose        values are 0 in last I elements of the column vector, where l is        an integer greater than or equal to 0 and less than or equal to        L−1, and

${{\sum\limits_{l = 0}^{L - 1}\;( {N_{l}^{0} + N_{l}^{1}} )} = N};$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T_(l) of X_(i,l)        ¹ whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where l is an integer greater than or equal to 0        and less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}\; T_{l}} = {N.}$

It should be noted that the indication information included in the CSImay be used to indicate that W₂ includes N X_(i,l) ¹ whose values are 0,or the indication information may be used to indicate that W₂ includes MX_(i,l) ¹ whose values are not 0. Because the network device has known atotal quantity of X_(i,l) ¹ included in W₂, after receiving theindication information used to indicate that W₂ includes M X_(i,l) ¹whose values are not 0, the network device can obtain, throughcalculation based on the indication information and the total quantityof X_(i,l) ¹ included in W₂, that W₂ includes N X_(i,l) ¹ whose valuesare 0.

In this way, the terminal device can report the indication informationto the network device. The indication information may be implemented ina plurality of forms.

In a possible implementation, before the sending, by the terminaldevice, a signal including the CSI to a network device, the methodfurther includes:

-   -   separately encoding, by the terminal device, the indication        information and the PMI2, to obtain the signal including the        CSI, or separately encoding the RI and the PMI2, to obtain the        signal including the CSI.

In other words, neither the indication information and the PMI2 nor theRI and the PMI2 can be encoded together in a joint encoding manner. Inthis way, it can be ensured that the network device can determine thePMI2 based on the RI and the indication information.

In a possible implementation, before the sending, by the terminaldevice, a signal including the CSI to a network device, the methodfurther includes:

-   -   encoding, by the terminal device, the RI and the indication        information in a joint encoding manner, to obtain the signal        including the CSI.

The encoding, by the terminal device, the RI and the indicationinformation in a joint encoding manner, to obtain the signal includingthe CSI may be implemented by using the following two methods:

A first method includes: representing the RI by using Q1 bits, andrepresenting the indication information by using Q2 bits;

-   -   combining, by the terminal device, the Q1 bits and the Q2 bits        into Q1+Q2 bits; and    -   encoding, by the terminal device, the Q1+Q2 bits, to obtain the        signal including the CSI.

A second method includes: selecting, by the terminal device, a statusvalue that is used to indicate combination information of the RI and theindication information; and

-   -   encoding, by the terminal device, the selected status value, to        obtain the signal including the CSI.

In the second method, a quantity of bits required to carry the statusvalue is less than a sum of a quantity of bits required to carry the RIand a quantity of bits required to carry the indication information.Therefore, compared with the method for separately carrying the RI andthe indication information by using bits, according to the method forjointly indicating the RI and the indication information by using thestatus value, a quantity of bits required to indicate the RI and theindication information can be reduced. In this way, resource overheadsrequired by the terminal device to feed back the CSI to the networkdevice are reduced.

In a possible implementation, the CSI further includes:

-   -   a first precoding matrix indicator where the PMI is used to        indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, in is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1.

In this way, after receiving the RI, the PMI1, and the PMI2, the networkdevice can determine W by using the three pieces of information.

According to a second aspect, an embodiment of this application providesa CSI receiving method, including:

-   -   receiving, by a network device, a signal that includes CSI and        that is sent by a terminal device, where the CSI includes an RI,        indication information, and a second precoding matrix indicator        PMI2;    -   obtaining, by the network device, the RI and the indication        information based on the signal including the CSI;    -   obtaining, by the network device, the PMI2 based on the RI and        the indication information; and    -   determining, by the network device, a precoding matrix W based        on the RI and the second precoding matrix indicator PMI2, where    -   W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L        columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a        matrix with 2I rows and L columns. N_(t) is a quantity of        antenna ports, L is a rank indicated by the RI, N_(t) is greater        than or equal to and I is an integer greater than or equal to 1;        an element at a location in an i^(th) row and an l^(th) column        in W₂ is Y_(i,l), i is an integer greater than or equal to 0 and        less than or equal to 2I−1, l is an integer greater than or        equal to 0 and less than or equal to L−1, and Y_(i,l) meets a        formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³ is a        complex number with modulus 1; and    -   the indication information is used to indicate that W₂ includes        N whose values are 0, the PMI2 is used to indicate a parameter        of W₂, and the parameter of W₂ indicated by the PMI2 includes        all X_(i,l) ¹ in W₂ and X_(i,l) ² and X_(i,l) ³, which are        corresponding to X_(i,l) ¹ other than the N X_(i,l) ¹ whose        values are 0, in W₂, and does not include X_(i,l) ² and X_(i,l)        ³, which are corresponding to the N X_(i,l) ¹ whose values are        0, in W₂.

X_(i,l) ¹ represents a wideband amplitude, X_(i,l) ² represents asubband amplitude, and X_(i,l) ³ represents a phase.

Alternatively, an embodiment of this application provides a CSIreceiving method, including:

-   -   receiving, by a network device, a signal that includes CSI and        that is sent by a terminal device, where the CSI includes an RI,        indication information, and a second precoding matrix indicator        PMI2;    -   obtaining, by the network device, the RI and the indication        information based on the signal including the CSI;    -   obtaining, by the network device, the PMI2 based on the RI and        the indication information; and    -   determining, by the network device, a precoding matrix W based        on the RI and the second precoding matrix indicator PMI2, where    -   W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L        columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a        matrix with 2I rows and L columns. N_(t) is a quantity of        antenna ports, L is a rank indicated by the RI, N_(t) is greater        than or equal to L, and I is an integer greater than or equal to        I; an element at a location in an i^(th) row and an l^(th)        column in W₂ is Y_(i,l), i is an integer greater than or equal        to 0 and less than or equal to 2I−1, is an integer greater than        or equal to 0 and less than or equal to L−1, Y_(i,l) meets a        formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, and X_(i,l) ³ is a complex        number with modulus 1; and    -   the indication information is used to indicate that W₂ includes        N X_(i,l) ¹ whose values are 0, the PMI2 is used to indicate a        parameter of W₂, and the parameter of W₂ indicated by the PMI2        includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding to        X_(i,l) ¹ other than the N X_(i,l) ¹ whose values are 0, in W₂,        and does not include X_(i,l) ³, corresponding to the N X_(i,l) ¹        whose values are 0, in W₂.

X_(i,l) ¹ represents a wideband amplitude, and X_(i,l) ³ represents aphase.

In the foregoing methods, the CSI that is sent by the terminal deviceand received by the network device includes the RI, the indicationinformation, and the PMI2, so that the network device can obtain the PM2by using the RI and the indication information, to determine W. In thescenario of a high precision codebook-based precoding matrix, in theprior art, a PMI2 that is sent by a terminal device to a network deviceneeds to indicate a parameter of all elements of W₂. However, in theforegoing solutions, the parameter of W₂, indicated by the PMI2 that issent by the terminal device to the network device, is a part ofparameters of elements of W₂. Therefore, a quantity of bits required bythe terminal device to send the PMI2 to the network device is reduced.The indication information is added to the CSI that is sent by theterminal device to the network device, so that the network device canobtain the PMI2 by using the RI and the indication information.Therefore, according to the foregoing methods, resource overheadsrequired by the terminal device to feed back the CSI to the networkdevice can be reduced in the scenario of a high precision codebook-basedprecoding matrix.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂, where l is an integer greater than or equal to 0 and less        than or equal to L−1, and

${{\sum\limits_{l = 0}^{L - 1}\; N_{l}} = N};$or

-   -   the indication information includes a quantity N_(l) ⁰ of        X_(i,l) ¹ whose values are 0 in first I elements of an column        vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose values        are 0 in last I elements of the column vector, where l is an        integer greater than or equal to 0 and less than or equal to        L−1, and

${{\sum\limits_{l = 0}^{L - 1}\;( {N_{l}^{0} + N_{l}^{1}} )} = N};$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T_(l) of X_(i,l)        ¹ whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where l is an integer greater than or equal to 0        and less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}\; T_{l}} = {N.}$

In a possible implementation, the obtaining, by the network device, theRI and the indication information based on the signal including the CSIincludes:

-   -   decoding, by the network device, bits that are in the signal        including the CSI and that are used to carry the RI and the        indication information, to obtain the RI and the indication        information; and    -   the obtaining, by the network device, the PMI2 based on the RI        and the indication information includes:    -   decoding, by the network device based on the RI and the        indication information, a bit that is in the signal including        the CSI and that is used to carry the PMI2, to obtain the PMI2.

In a possible implementation, the decoding, by the network device basedon the RI and the indication information, a bit that is in the signalincluding the CSI and that is used to carry the PMI2, to obtain the PMI2includes:

-   -   determining, by the network device based on the RI and the        indication information, a quantity of bits required to decode        the PMI2; and    -   decoding, by the network device based on the RI and the quantity        of bits, the bit that is used to carry the PMI2, to obtain the        PMI2.

In a possible implementation, the decoding, by the network device, bitsthat include the RI and the indication information and that are in theCSI signal, to obtain the RI and the indication information includes:

-   -   decoding, by the network device based on a quantity Q1+Q2 of        bits, a signal that includes the RI and the indication        information and that is in the CSI signal, to obtain the RI and        the indication information, where    -   the RI is represented by using Q1 bits, and the indication        information is represented by using Q2 bits.

In a possible implementation, the decoding, by the network device, bitsthat are in the signal including the CSI and that are used to carry theRI and the indication information, to obtain the RI and the indicationinformation includes:

-   -   obtaining, by the network device, a status value based on the        bits that are used to carry the RI and the indication        information, where the status value is used to indicate        combination information of the RI and the indication        information; and    -   obtaining, by the network device, the RI and the indication        information based on the status value.

In a possible implementation, the CSI further includes:

-   -   a first precoding matrix indicator PMI1, where the PMI is used        to indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1; and

-   -   the determining, by the network device, W based on the RI and        the PMI2 includes:    -   determining, by the network device, W based on the RI, the PMI1        and the PMI2.

According to a third aspect, an embodiment of this application providesa terminal device, including:

-   -   a processing unit, configured to determine a precoding matrix W,        where    -   W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L        columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a        matrix with 2I rows and L columns. N_(t) is a quantity of        antenna ports, L is a rank indicated by an RI, N_(t) is greater        than or equal to L, and I is an integer greater than or equal to        1; an element at a location in an i^(th) row and an l^(th)        column in W₂ is Y_(i,l), i is an integer greater than or equal        to 0 and less than or equal to 2I−1, l is an integer greater        than or equal to 0 and less than or equal to L−1, and Y_(i,l)        meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³        is a complex number with modulus 1;    -   the processing unit is further configured to generate CSI that        includes the RI, indication information, and a second precoding        matrix indicator PMI2; and    -   the indication information is used to indicate that W₂ includes        N X_(i,l) ¹ whose values are 0, the PMI2 is used to indicate a        parameter of W₂ and the parameter of W₂ indicated by the PMI2        includes all X_(i,l) ¹ in W₂ and X_(i,l) ² and X_(i,l) ³, which        are corresponding to X_(i,l) ¹ other than the N X_(i,l) ¹ whose        values are 0, in W₂, and does not include X_(i,l) ² and X_(i,l)        ³, which are corresponding to the N X_(i,l) ¹ whose values are        0, in W₂; and    -   a transceiver unit, configured to send a signal including the        CSI to a network device.

X_(i,l) ¹ represents a wideband amplitude, X_(i,l) ² represents asubband amplitude, and X_(i,l) ³ represents a phase.

Alternatively, an embodiment of this application provides a terminaldevice, including:

-   -   a processing unit, configured to determine a precoding matrix W,        where    -   W meets a formula W=W₁×X₂, W is a matrix with N_(t) rows and L        columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a        matrix with 2I rows and L columns. N_(t) is a quantity of        antenna ports, L is a rank indicated by an RI, N_(t) is greater        than or equal to L, and I is an integer greater than or equal to        1; an element at a location in an i^(th) row and an l^(th)        column in W₂ is Y_(i,l), i is an integer greater than or equal        to 0 and less than equal to 2I−1, l is an integer greater than        or equal to 0 and less than or equal to L−1, and Y_(i,l) meets a        formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is a complex        number with modulus 1;    -   the processing unit is further configured to generate CSI that        includes the RI, indication information, and a second precoding        matrix indicator PMI2; and    -   the indication information is used to indicate that W₂ includes        N X_(i,l) ¹ whose values are 0, the PMI2 is used to indicate a        parameter of W₂, and the parameter of W₂ indicated by the PMI2        includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding to        X_(i,l) ¹ other than the N X_(i,l) ¹ whose values are 0, in W₂,        and does not include X_(i,l) ³, corresponding to the N X_(i,l) ¹        whose values are 0, in W₂; and    -   a transceiver unit, configured to send a signal including the        CSI to a network device.

X_(i,l) ¹ represents a wideband amplitude, and X_(i,l) ³ represents aphase.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂, where l is an integer greater than or equal to 0 and less        than or equal to L−1, and

${{\sum\limits_{l = 0}^{L - 1}\; N_{l}} = N};$or

-   -   the indication information includes a quantity N_(l) ⁰ of        X_(i,l) ¹ whose values are 0 in first I elements of an l^(th)        column vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose        values are 0 in last I elements of the column vector, where l is        an integer greater than or equal to 0 and less than or equal to        L−1, and

${{\sum\limits_{l = 0}^{L - 1}\;( {N_{l}^{0} + N_{l}^{1}} )} = N};$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T_(l) of X_(i,l)        ¹ whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where l is an integer greater than or equal to 0        and less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}\; T_{l}} = {N.}$

In a possible implementation, the processing unit is further configuredto:

-   -   before the transceiver unit sends the signal including the CSI        to the network device, separately encode the indication        information and the PMI2, to obtain the signal including the        CSI.

In a possible implementation, the processing unit is further configuredto:

before the transceiver unit sends the signal including the CSI to thenetwork device, encode the RI and the indication information in a jointencoding manner, to obtain the signal including the CSI.

In a possible implementation, when encoding the RI and the indicationinformation in the joint encoding manner, to obtain the signal includingthe CSI, the processing unit is specifically configured to:

-   -   represent the RI by using Q1 bits, and represent the indication        information by using Q2 bits;    -   combine the Q1 bits and the Q2 bits into Q1+Q2 bits; and    -   encode the Q1+Q2 bits, to obtain the signal including the CSI.

In a possible implementation, when encoding the RI and the indicationinformation in the joint encoding manner, to obtain the signal includingthe CSI, the processing unit is specifically configured to:

-   -   select a status value that is used to indicate combination        information of the RI and the indication information; and    -   encode the selected status value, to obtain the signal including        the CSI.

In a possible implementation, the CSI further includes:

-   -   a first precoding matrix indicator PMI1, where the PMI is used        to indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1.

According to a fourth aspect, an embodiment of this application providesa network device, including:

-   -   a transceiver unit, configured to receive a signal that includes        CSI and that is sent by a terminal device, where the CSI        includes an RI, indication information, and a second precoding        matrix indicator PMI2; and    -   a processing unit, configured to obtain the RI and the        indication information based on the signal including the CSI;        obtain the PMI2 based on the RI and the indication information;        and determine a precoding matrix W based on the RI and the        second precoding matrix indicator PMI2, where    -   W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L        columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a        matrix with 2I rows and L columns, N_(t) is a quantity of        antenna ports, L is a rank indicated by the RI, N_(t) is greater        than or equal to L, and I is an integer greater than or equal to        1; an element at a location in an i^(th) row and an l^(th)        column in W₂ is Y_(i,l), i is an integer greater than or equal        to 0 and less than or equal to 2I−1, l is an integer greater        than or equal to 0 and less than or equal to L−1, and Y_(i,l)        meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, and        X_(i,l) ³ and is a complex number with modulus 1; and    -   the indication information is used to indicate that W₂ includes        N X_(i,l) ¹ whose values are 0, the PMI2 is used to indicate a        parameter of W₂ and the parameter of W₂ indicated by the PMI2        includes all X_(i,l) ¹ in W₂ and X_(i,l) ² and X_(i,l) ³, which        are corresponding to X_(i,l) ¹ other than the N X_(i,l) ¹ whose        values are 0, in W₂, and does not include X_(i,l) ² and X_(i,l)        ³, which are corresponding to the N X_(i,l) ¹ whose values are        0, in W₂.

X_(i,l) ¹ represents a wideband amplitude, X_(i,l) ² represents asubband amplitude, and X_(i,l) ³ represents a phase.

Alternatively, an embodiment of this application provides a networkdevice, including:

-   -   a transceiver unit, configured to receive a signal that includes        CSI and that is sent by a terminal device, where the CSI        includes an RI, indication information, and a second precoding        matrix indicator PMI2; and    -   a processing unit, configured to obtain the RI and the        indication information based on the signal including the CSI;        obtain the PMI2 based on the RI and the indication information;        and determine a precoding matrix W based on the RI and the        second precoding matrix indicator PMI2, where    -   W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L        columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a        matrix with 2I rows and L columns. N_(t) is a quantity of        antenna ports, L is a rank indicated by the RI, N_(t) is greater        than or equal to L, and I is an integer greater than or equal to        1; an element at a location in an i^(th) row and an l^(th)        column in W₂ is Y_(i,l), i is an integer greater than or equal        to 0 and less than or equal to 2I−1, l is an integer greater        than or equal to 0 and less than or equal to L−1, and Y_(i,l)        meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is a        complex number with modulus I; and    -   the indication information is used to indicate that W₂ includes        N X_(i,l) ¹ whose values are 0, the PMI2 is used to indicate a        parameter of W₂ and the parameter of W₂ indicated by the PMI2        includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding to        X_(i,l) ¹ other than the N X_(i,l) ¹ whose values are 0, in W₂,        and does not include X_(i,l) ³, corresponding to the N X_(i,l) ¹        whose values are 0, in W₂.

In a possible implementation, X_(i,l) ¹ represents a wideband amplitude,and X_(i,l) ³ represents a phase.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂ where 1 is an integer greater than or equal to 0 and less        than or equal to L−1, and

${{\sum\limits_{l = 0}^{L - 1}\; N_{l}} = N};$or

-   -   the indication information includes a quantity N_(l) ⁰ of        X_(i,l) ¹ whose values are 0 in first I elements of an l^(th)        column vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose        values are 0 in last I elements of the column vector, where l is        an integer greater than or equal to 0 and less than or equal to        L−1, and

${{\sum\limits_{l = 0}^{L - 1}\;( {N_{l}^{0} + N_{l}^{1}} )} = N};$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T_(l) of X_(i,l)        ¹ whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where l is an integer greater than or equal to 0        and less than or equal to L−1, and

$\begin{matrix}{{\sum\limits_{l = 0}^{L - 1}\; T_{l}} = {N.}} & \;\end{matrix}$

In a possible implementation, when obtaining the RI and the indicationinformation based on the signal including the CSI, the processing unitis specifically configured to:

-   -   decode bits that are in the signal including the CSI and that        are used to carry the RI and the indication information, to        obtain the RI and the indication information; and    -   when obtaining the PMI2 based on the RI and the indication        information, the processing unit is specifically configured to:    -   decode, based on the RI and the indication information, a bit        that is in the signal including the CSI and that is used to        carry the PMI2, to obtain the PMI2.

In a possible implementation, when decoding, based on the RI and theindication information, the bit that is in the signal including the CSIand that is used to carry the PMI2, to obtain the PMI2, the processingunit is specifically configured to:

-   -   determine, based on the RI and the indication information, a        quantity of bits required to decode the PMI2; and    -   decode, based on the RI and the quantity of bits, the bit that        is used to carry the PMI2, to obtain the PMI2.

In a possible implementation, when decoding the bits that include the RIand the indication information and that are in the CSI signal, to obtainthe RI and the indication information, the processing unit isspecifically configured to:

decode, based on a quantity Q1+Q2 of bits, a signal that includes the RIand the indication information and that is in the CSI signal, to obtainthe RI and the indication information, where

-   -   the RI is represented by using Q1 bits, and the indication        information is represented by using Q2 bits.

In a possible implementation, when decoding the bits that are in thesignal including the CSI and that are used to carry the RI and theindication information, to obtain the RI and the indication information,the processing unit is specifically configured to:

-   -   obtain a status value based on the bits that are used to carry        the RI and the indication information, where the status value is        used to indicate combination information of the RI and the        indication information; and    -   obtain the RI and the indication information based on the status        value.

In a possible implementation, the CSI further includes:

-   -   a first preceding matrix indicator PMI1, where the PMI is used        to indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1; and

-   -   when determining W based on the RI and the PMI2, the processing        unit is specifically configured to:    -   determine W based on the RI, the PMI1, and the PMI2.

According to a fifth aspect, an embodiment of this application furtherprovides a terminal device, where the terminal device has functions ofimplementing actions of the terminal device in the method example of thefirst aspect. The functions may be implemented by using hardware. Astructure of the terminal device includes a memory, a processor, and atransceiver; the memory is configured to store a computer-readableprogram; the processor invokes an instruction stored in the memory, toperform the method according to any one of the implementations of thefirst aspect, to implement a function of the processing unit included inthe structure of the terminal device in the third aspect; and thetransceiver is configured to receive and/or send data under control ofthe processor, to implement a function of the transceiver unit includedin the structure of the terminal device in the third aspect.

According to a sixth aspect, an embodiment of this application furtherprovides a computer storage medium, where the storage medium stores asoftware program, and when being read and executed by one or moreprocessors, the software program is capable of implementing the methodaccording to the first aspect or any one of the implementations of thefirst aspect.

According to a seventh aspect, an embodiment of this application furtherprovides a network device, where the network device has functions ofimplementing actions of the network device in the method example of thesecond aspect. The functions may be implemented by using hardware. Astructure of the network device includes a memory, a processor, and atransceiver; the memory is configured to store a computer-readableprogram; the processor invokes an instruction stored in the memory, toperform the method according to any one of the implementations of thesecond aspect, to implement a function of the processing unit includedin the structure of the network device in the fourth aspect; and thetransceiver is configured to receive and/or send data under control ofthe processor, to implement a function of the transceiver unit includedin the structure of the network device in the fourth aspect.

According to an eighth aspect, an embodiment of this application furtherprovides a computer storage medium, where the storage medium stores asoftware program, and when being read and executed by one or moreprocessors, the software program is capable of implementing the methodaccording to the second aspect or any one of the implementations of thesecond aspect.

According to a ninth aspect, an embodiment of this application furtherprovides a communications system. The communications system includes aterminal device and a network device. The terminal device is configuredto perform the method according to the first aspect or any one of theimplementations of the first aspect, and the network device isconfigured to perform the method according to the second aspect or anyone of the implementations of the second aspect.

According to the technical solutions provided in the embodiments of thisapplication, resource overheads required by the terminal device to feedback the CSI to the network device can be reduced in the scenario of ahigh precision codebook-based precoding matrix.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic architectural diagram of an NR system in anembodiment of this application;

FIG. 2 is a schematic flowchart of a channel state information sending,receiving method according to an embodiment of this application;

FIG. 3 is a schematic diagram of periodically reporting CSI according toan embodiment of this application;

FIG. 4 is a schematic structural diagram of a terminal device accordingto an embodiment of this application;

FIG. 5 is a schematic structural diagram of another terminal deviceaccording to an embodiment of this application;

FIG. 6 is a schematic structural diagram of a network device accordingto an embodiment of this application;

FIG. 7 is a schematic structural diagram of another network deviceaccording to an embodiment of this application; and

FIG. 8 is a schematic structural diagram of another communicationssystem according to an embodiment of this application.

DESCRIPTION F EMBODIMENTS

Embodiments of this application provide a channel state informationsending method, a channel state information receiving method, and adevice, to reduce resource overhead required when a terminal devicefeeds back CSI to a network device in a scenario of a high precisioncodebook-based precoding matrix. The methods and the device are based ona same invention concept. Because principles of resolving problemsaccording to the methods and the device are similar, mutual referencemay be made to implementations of the device and the methods, andrepeated content is not described.

The technical solutions provided in the embodiments of this applicationare applicable to a wireless communications system that applies MIMOtechnologies, such as an LTE system or an NR system and the like. In theMIMO technologies, a transmit end and a receive end each use a pluralityof transmit antennas and receive antennas, so that signals are sent andreceived by using the plurality of antennas of the transmit end and thereceive end, thereby improving communication quality. According to theMIMO technologies, spatial resources can be utilized well,multiple-transmit multiple-receive is implemented by using a pluralityof antennas, and a system channel capacity is multiplied without addingspectrum resources or increasing antenna transmit power.

In a MIMO system, if a network device can obtain all or a part ofdownlink channel information, a precoding technology can be used toimprove signal transmission quality and increase a signal transmissionrate. The technical solutions provided in the embodiments of thisapplication are applicable to a scenario in which a terminal devicefeeds back CSI to a network device and the network device estimates aprecoding matrix for a downlink channel based on the CSI. In thisscenario, the terminal device measures the downlink channel based on acommon reference signal (CRS), to obtain a channel matrix; and theterminal device may select, from a preset codebook according to anoptimization rule, a precoding matrix that best matches the downlinkchannel, and further determines a precoding matrix indicator PMI, andfeed back the PMI to the network device as CSI. The terminal device mayfurther determine, based on the determined PMI, a channel qualityachieved after the PMI is used, i.e., a CQI. The CQI is also fed back tothe network device as the CSI. The following describes the precodingmatrix used in the embodiments of this application.

In design of a communications system, a codebook may include a pluralityof precoding matrices, and content of the codebook is known to both of atransmitter and a receiver.

In the embodiments of this application, if a complex number represents aphase, and the complex number is a complex number with modulus 1,multiplying the complex number and another complex number (for example,a complex number A) would only change the phase of the complex number Abut the amplitude of the complex number A is unchanged.

The precoding matrix in the embodiments of this application is for ahigh precision codebook-based precoding matrix defined in the standardLTE system release 14 and an NR system. The precoding matrix uses adual-stage codebook feedback mechanism to reduce feedback load. To bespecific, a precoding matrix (or referred to as a precoding vector) W isa product of a first-stage feedback matrix W₁ and a second-stagefeedback matrix W₂. W may be represented by Formula 1:W=W ₁ ×W ₂  Formula 1

In Formula 1, W is a matrix with N_(t) rows and L columns, N_(t) is thequantity of antenna ports, L is a rank of a channel matrix, i.e., a rankindicated by RI, and N_(t) is greater than or equal to L. W₁ is a blockdiagonal matrix with N_(t) rows and 2I columns, I is an integer greaterthan or equal to 1 and may represent the quantity of beam vectorsincluded in each diagonal matrix of W₁, and W₁ may be represented byFormula 2:

$\begin{matrix}{W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}} & {{Formula}\mspace{14mu} 2}\end{matrix}$

In Formula 2, X₁ is a matrix with N_(t)/2 rows and I columns, X₁ meetsX₁=[v₀ . . . v_(M-1)], v_(m) is a column vector including N_(t)/2elements, v_(m) represents a beam vector, m is an integer greater thanor equal to 0 and less than or equal to I−1, and I is an integer greaterthan or equal to 1.

In Formula 1, W₂ is a matrix with 2I rows and L columns, I is an integergreater than 1, L is the rank of the channel matrix, i.e., the rankindicated by RI, and an element Y_(i,l) at a location in an i^(th) rowand an l^(th) column in W₂ may be represented by two formulas.

First case: The element Y_(i,l) at the location in the i^(th) row andthe l^(th) column in W₂ meets Formula 3:Y _(i,l) =X _(i,l) ¹ ×X _(i,l) ² ×X _(i,l) ³  Formula 3

In Formula 3, i is an integer greater than or equal to 0 and less thanor equal to 2I−1, l is an integer greater than or equal to 0 and lessthan or equal to L−1, X_(i,l) ¹ represents a wideband amplitude of achannel, X_(i,l) ² represents a subband amplitude of the channel,X_(i,l) ³ represents a phase of the channel, and X_(i,l) ³ is a complexnumber with modulus 1. When Y_(i,l) meets Formula 3, X_(i,l) ¹ inY_(i,l) which is at any location in W₂ corresponds to X_(i,l) ² andX_(i,l) ³ in Y_(i,l). For X_(i,l) ¹, X_(i,l) ², and X_(i,l) ³, andX_(i,l) ³ that have correspondence, a value of X_(i,l) ¹ may determine avalue of X_(i,l) ² and a value of X_(i,l) ³. When the value of X_(i,l) ¹is 0, the value of X_(i,l) ² corresponding to X_(i,l) ¹ is 0, and thevalue of X_(i,l) ³ corresponding to X_(i,l) ¹ is also 0. For example, inan element Y_(2,1)=X_(2,1) ¹×X_(2,1) ²×X_(2,1) ³ at location in a 2^(nd)row and a 1^(st) column in W₂, X_(2,1) ¹ corresponds to X_(2,1) ² andX_(2,1) ³. When a value of X_(2,2) ¹ is 0, Y_(2,1)=0. A Value of X_(2,1)² corresponding to X_(2,1) ¹ does not affect a value of Y_(2,1), and avalue of X_(2,1) ³ corresponding to X_(2,1) ¹ does not affect the valueof Y_(2,1), either. When Y_(i,l) meets Formula 3, W₂ may be representedby the following two formulas.

When the rank is 1, W₂ may be represented by Formula 4:

$\begin{matrix}{W_{2} = \begin{bmatrix}{p_{0,0,0}^{({WB})} \cdot p_{0,0,0}^{({SB})} \cdot c_{0,0,0}} \\{p_{0,0,1}^{({WB})} \cdot p_{0,0,1}^{({SB})} \cdot c_{0,0,1}} \\\vdots \\{p_{0,0,{I - 1}}^{({WB})} \cdot p_{0,0,{I - 1}}^{({SB})} \cdot c_{0,0,{I - 1}}} \\{p_{1,0,0}^{({WB})} \cdot p_{1,0,0}^{({SB})} \cdot c_{1,0,0}} \\{p_{1,0,1}^{({WB})} \cdot p_{1,0,1}^{({SB})} \cdot c_{1,0,1}} \\\vdots \\{p_{1,0,{I - 1}}^{({WB})} \cdot p_{1,0,{I - 1}}^{({SB})} \cdot c_{1,0,{I - 1}}}\end{bmatrix}} & {{Formula}\mspace{14mu} 4}\end{matrix}$

When the rank is 2, W₂ may be represented by Formula 5:

$\begin{matrix}{W_{2} = \begin{bmatrix}{p_{0,0,0}^{({WB})} \cdot p_{0,0,0}^{({SB})} \cdot c_{0,0,0}} & {p_{0,1,0}^{({WB})} \cdot p_{0,1,0}^{({SB})} \cdot c_{0,1,0}} \\{p_{0,0,1}^{({WB})} \cdot p_{0,0,1}^{({SB})} \cdot c_{0,0,1}} & {p_{0,1,1}^{({WB})} \cdot p_{0,1,1}^{({SB})} \cdot c_{0,1,1}} \\\vdots & \vdots \\{p_{0,0,{I - 1}}^{({WB})} \cdot p_{0,0,{I - 1}}^{({SB})} \cdot c_{0,0,{I - 1}}} & {p_{0,1,{I - 1}}^{({WB})} \cdot p_{0,1,{I - 1}}^{({SB})} \cdot c_{0,1,{I - 1}}} \\{p_{1,0,0}^{({WB})} \cdot p_{1,0,0}^{({SB})} \cdot c_{1,0,0}} & {p_{1,1,0}^{({WB})} \cdot p_{1,1,0}^{({SB})} \cdot c_{1,1,0}} \\{p_{1,0,1}^{({WB})} \cdot p_{1,0,1}^{({SB})} \cdot c_{1,0,1}} & {p_{1,1,0}^{({WB})} \cdot p_{1,1,0}^{({SB})} \cdot c_{1,1,0}} \\\vdots & \vdots \\{p_{1,0,{I - 1}}^{({WB})} \cdot p_{1,0,{I - 1}}^{({SB})} \cdot c_{1,0,{I - 1}}} & {p_{1,1,{I - 1}}^{({WB})} \cdot p_{1,1,{I - 1}}^{({SB})} \cdot c_{1,1,{I - 1}}}\end{bmatrix}} & {{Formula}\mspace{14mu} 5}\end{matrix}$

In Formula 4 and Formula 5, an element at any location in W₂ may berepresented as p_(r,l,m) ^((WB))·p_(r,l,m) ^((SB))·c_(r,l,m). p_(r,l,m)^((WB)) represents X_(i,l) ¹, the wideband amplitude of the channel, andp_(r,l,m) ^((WB))∈{1, √{square root over (0.5)} √{square root over(0.25)} √{square root over (0.125)} √{square root over (0.0625)}√{square root over (0.0313)} √{square root over (0.0313)} √{square rootover (0.0156)} 0}; p_(r,l,m) ^((SB)) represents X_(i,l) ², the subbandamplitude of the channel, and p_(r,l,m) ^((SB))∈{1 √{square root over(0.5)}}; and c_(r,l,m) represents X_(i,l) ³, the phase of the channel,and

$c_{r,l,m} \in {\{ {e^{j\;\frac{n}{2}\pi},{n = 0},1,2,3} \}\mspace{14mu}{or}\mspace{14mu} c_{r,l,m}} \in {\{ {e^{j\;\frac{n}{2}\pi},{n = 0},1,2,3,\ldots\mspace{14mu},7} \}.}$r represents an indicator of an antenna polarization directiondimension, l represents a sequence number of a data layer, and irepresents a sequence number of a beam vector b_(i) ^(m) in W₁.

Second case: The element Y_(i,l) at the location in the i^(th) row andthe l^(th) column in W₂ meets Formula 6:Y _(i,l) =X _(i,l) ¹ ×X _(i,l) ³  Formula 6

In Formula 6, i is an integer greater than 0 and less than 2I−1, l is aninteger greater than 0 and less than L−1, X_(i,l) ¹ represents awideband amplitude of a channel, X_(i,l) ³ represents a phase of thechannel, and X_(i,l) ³ is a complex number with modulus 1. When Y_(i,l)meets Formula 6, X_(i,l) ¹ in Y_(i,l) which is at any location in W₂corresponds to X_(i,l) ³ in Y_(i,l). For X_(i,l) ¹ and X_(i,l) ³ have acorrespondence, a value of X_(i,l) ¹ may determine a value of x_(i,l) ³.When the value of X_(i,l) ¹ is 0, the value of X_(i,l) ³ correspondingto X_(i,l) ¹ is 0. For example, in an element Y_(2,1)=X_(2,1) ¹×X_(2,1)³ at a location in a 2^(nd) row and a 1^(st) column in W₂, when a valueof X_(2,1) ¹ is 0, Y_(2,1)=0. A value of X_(2,1) ³ corresponding toX_(2,1) ¹ does not affect a value of Y_(2,1). When Y_(i,l) meets Formula6, W₂ may be represented by the following two formulas.

When the rank is 1, W₂ may be represented by Formula 7:

$\begin{matrix}{W_{2} = \begin{bmatrix}{p_{0,0,0}^{({WB})} \cdot c_{0,0,0}} \\{p_{0,0,1}^{({WB})} \cdot c_{0,0,1}} \\\vdots \\{p_{0,0,{I - 1}}^{({WB})} \cdot c_{0,0,{I - 1}}} \\{p_{1,0,0}^{({WB})} \cdot c_{1,0,0}} \\{p_{1,0,1}^{({WB})} \cdot c_{1,0,1}} \\\vdots \\{p_{1,0,{I - 1}}^{({WB})} \cdot c_{1,0,{I - 1}}}\end{bmatrix}} & {{Formula}\mspace{14mu} 7}\end{matrix}$

When the rank is 2, W₂ may be represented by Formula 8:

$\begin{matrix}{W_{2} = \begin{bmatrix}{p_{0,0,0}^{({WB})} \cdot c_{0,0,0}} & {p_{0,1,0}^{({WB})} \cdot c_{0,1,0}} \\{p_{0,0,1}^{({WB})} \cdot c_{0,0,1}} & {p_{0,1,1}^{({WB})} \cdot c_{0,1,1}} \\\vdots & \vdots \\{p_{0,0,{I - 1}}^{({WB})} \cdot c_{0,0,{I - 1}}} & {p_{0,1,{I - 1}}^{({WB})} \cdot c_{0,1,{I - 1}}} \\{p_{1,0,0}^{({WB})} \cdot c_{1,0,0}} & {p_{1,1,0}^{({WB})} \cdot c_{1,1,0}} \\{p_{1,0,1}^{({WB})} \cdot c_{1,0,1}} & {p_{1,1,0}^{({WB})} \cdot c_{1,1,0}} \\\vdots & \vdots \\{p_{1,0,{I - 1}}^{({WB})} \cdot c_{1,0,{I - 1}}} & {p_{1,1,{I - 1}}^{({WB})} \cdot c_{1,1,{I - 1}}}\end{bmatrix}} & {{Formula}\mspace{14mu} 8}\end{matrix}$

In Formula 7 and Formula 8, an element at any location in W₂ may berepresented as p_(r,l,m) ^((WB))·c_(r,l,m). p_(r,l,m) ^((WB)) representsX_(i,l) ¹, the wideband amplitude of the channel, and p_(r,l,m)^((WB))∈{1, √{square root over (0.5)} √{square root over (0.25)}√{square root over (0.125)} √{square root over (0.0625)} √{square rootover (0.0313)} √{square root over (0.0313)} √{square root over (0.0156)}0}; and c_(r,l,m)

$c_{r,l,m} \in {\{ {e^{j\;\frac{n}{2}\pi},{n = 0},1,2,3} \}\mspace{14mu}{or}\mspace{14mu} c_{r,l,m}} \in {\{ {e^{j\;\frac{n}{2}\pi},{n = 0},1,2,3,\ldots\mspace{14mu},7} \}.}$r represents an indicator of an antenna polarization directiondimension, l represents a sequence number of a data layer, and irepresents a sequence number of a beam vector b_(l) ^(m) in W₁.

The technical solutions provided in the embodiments of this applicationmay be used in an NR system. For an architectural diagram of the NRsystem, refer to FIG. 1. The NR system includes at least one networkdevice, and at least one terminal device connected to each networkdevice. The technical solutions provided in the embodiments of thisapplication relate to the terminal device and the network device.

The terminal device may be a device that provides voice and/or dataconnectivity to a user, a handheld device having a wireless connectionfunction, or another processing device connected to a wireless modem.The wireless terminal device may communicate with one or more corenetworks through a RAN. The wireless terminal device may be a mobileterminal device, such as a mobile phone (also referred to as a“cellular” phone) or a computer with a mobile terminal device. Forexample, the wireless terminal device may be a portable, pocket-sized,handheld, computer built-in, or in-vehicle mobile apparatus, andexchanges voice and/or data with a radio access network. For example,the wireless terminal device may be a device such as a personalcommunications service (PCS) phone, a cordless telephone set, a SessionInitiation Protocol (SIP) phone, a wireless local loop (WLL) station, ora personal digital assistant (PDA). The wireless terminal device mayalso be referred to as a system, a subscriber unit, a subscriberstation, a mobile station, a mobile console (Mobile), a remote station,an access point, a remote terminal device, an access terminal device, auser terminal device, a user agent, a user device, or user equipment.

The network device may be a base station or an access point, or may be adevice that communicates with a wireless terminal device over an airinterface in an access network by using one or more sectors. The networkdevice may be configured to perform mutual conversion between a receivedover-the-air frame and an Internet Protocol (IP) packet, and serve as arouter between the wireless terminal device and a rest portion of theaccess network. The rest portion of the access network may include an IPnetwork. The network device may further coordinate management of an airinterface attribute. For example, the network device may be a networkdevice (BTS, Base Transceiver Station) a Global System for MobileCommunications (GSM) or Code Division Multiple Access (CDMA), a networkdevice (NodeB) in Wideband Code Division Multiple Access (WCDMA), or anevolved network device (evolutional Node B, eNB or e-NodeB) in LTE. Thisis not limited in the embodiments of the present invention.

The following describes the technical solutions provided in theembodiments of this application.

An embodiment of this application provides a CSI sending and receivingmethod. As shown in FIG. 2, the method includes the following steps.

S201. A terminal device determines a precoding matrix W.

W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L columns,W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a matrix with 2Irows and L columns. N_(t) is a quantity of antenna ports, L is a rankindicated by an RI, N_(t) is greater than or equal to L, and I is aninteger greater than or equal to 1; and an element at a location in ani^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³ orY_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³ is a complex numberwith modulus 1. For detailed descriptions about W, W₁, and W₂, refer tothe foregoing description. Details are not repeatedly described herein.

A method for determining W by the terminal device in S201 includes:determining, by the terminal device, a physical channel based on achannel state information-reference signal (CSI-RS) delivered by anetwork device, and then determining, based on the physical channel, Wfrom a predefined precoding matrix group. A principle for determining Wmay be: if the network device performs weighting on data based on theprecoding matrix W, a signal-to-noise ratio, a throughput, or spectrumefficiency for data received by the terminal device is the highest.

S202. The terminal device generates CSI that includes an RI, indicationinformation and a second precoding matrix indicator PMI2.

The RI is an indicator of a rank of a channel matrix, and the RI is usedby the terminal device to report, to the network device, the layerquantity of data that can be carried by the physical channel. Forexample, RI=0 represents that a current physical channel can carry dataof one layer. The indication information is used to indicate that W₂includes N X_(i,l) ¹ whose values are 0. The PMI2 is used to indicate aparameter of W₂. The following separately describes the CSI generated bythe terminal device in S202 when Y_(i,l) meets the formulaY_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³ and when Y_(i,l) meets the formulaY_(i,l)=X_(i,l) ¹×X_(i,l) ³.

Case 1: Y_(i,l) meets the formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³.

Based on Case 1, the indication information included in the CSI is usedto indicate that W₂ includes N X_(i,l) ¹ whose values are 0. This couldbe understood as follows: The indication information is used to indicatethat N X_(i,l) ¹ whose values are 0 are included in all elements of W₂,N represents a quantity of all X_(i,l) ¹ whose values are 0 included inW₂. For example, when the quantity of all X_(i,l) ¹ whose values are 0included in W₂ is 5, N is 5, the indication information is used toindicate that W₂ includes five X_(i,l) ¹ whose values are 0, and 3 bitsare required to carry the indication information. The PMI2 reported bythe network device is used to indicate all X_(i,l) ¹ in the precodingmatrix W₂ and those X_(i,l) ² and X_(i,l) ³ other than X_(i,l) ² andX_(i,l) ³ that correspond to the five X_(i,l) ¹ whose values are 0.Since a wideband reporting manner or a long-period reporting manner isused for X_(i,l) ¹, while a subband or short-period reporting manner isused for X_(i,l) ² and X_(i,l) ³, the quantity of bits required by thePMI2 that indicates W₂ can be reduced when the UE does not reportX_(i,l) ² and X_(i,l) ³ that correspond to X_(i,l) ¹ whose value is 0.

The indication information included in the CSI is used to indicate thatW₂ includes N X_(i,l) ¹ whose values are 0. This may be alternativelyunderstood as follows: The indication information is used to indicatethat N X_(i,l) ¹ whose values are 0 are included in a part of elementsW₂. If only a part of all X_(i,l) ¹ whose values are 0 in W₂ areincluded in the a part of elements of W₂, N represents a quantity of thea part of all X_(i,l) ¹ whose values are 0 included in W₂. For example,when a quantity of all X_(i,l) ¹ whose values are 0 included in W₂ is 5,N may be 0 to 4. Using N=4 as an example, the indication information isused to indicate that W₂ includes four X_(i,l) ¹ whose values are 0, and2 bits are required to carry the indication information. In animplementation, it may be determined, based on a preset order, that aquantity of which X_(i,l) ¹ in all X_(i,l) ¹ whose values are 0 in W₂ isthe quantity of X_(i,l) ¹, whose values are 0, indicated by theindication information. Both the terminal device and the network deviceknow the preset order. Using that W₂ includes one column vector asexample, assuming that the quantity of all X_(i,l) ¹ whose values are 0included in W₂ is 5, four X_(i,l) ¹ whose values are 0 in W₂ may besuccessively determined from top to down starting from a first rowvector in W₂. The four determined X_(i,l) ¹ whose values are 0 then arethe four X_(i,l) ¹ whose values are 0 indicated by the indicationinformation.

In conclusion, assuming that the quantity of all X_(i,l) ¹ whose valuesare 0 included in W₂ is 5, compared with that the indication informationis used to indicate that W₂ includes five X_(i,l) ¹ whose values are 0,when the indication information is used to indicate that W₂ includesfour X_(i,l) ¹ whose values are 0, fewer bits are required to carry theindication information. Therefore, when N represents the quantity of apart of all X_(i,l) ¹ whose values are 0 included in W₂, the quantity ofbits required to carry the indication information may be reduced.

Based on Case 1, the PMI2 included in the CSI is used to indicate aparameter of W₂ in W, and the parameter of W₂ indicated by the PMI2includes all X_(i,l) ¹ in W₂ and X_(i,l) ² and X_(i,l) ³, which arecorresponding to X_(i,l) ¹ other than the N X_(i,l) ¹ whose values are0, in W₂, and the parameter of W₂ does not include X_(i,l) ² and X_(i,l)³, which are corresponding to the N X_(i,l) ¹ whose values are 0, in W₂.That X_(i,l) ¹ corresponds to X_(i,l) ² and X_(i,l) ³ means that, basedon Case 1, X_(i,l) ¹ in Y_(i,l) at any location in W₂ corresponds toX_(i,l) ² and X_(i,l) ³ in Y_(i,l). For X_(i,l) ¹, X_(i,l) ², andX_(i,l) ³ that have a correspondence, a value of X_(i,l) ¹ may determinea value of X_(i,l) ² and a value of X_(i,l) ³. When the value of X_(i,l)¹ is 0, the value of X_(i,l) ² corresponding to X_(i,l) ¹ is 0, and thevalue of X_(i,l) ³ corresponding to X_(i,l) ¹ is also 0. For example, inan element Y_(2,1)=X_(2,1) ¹×X_(2,1) ²×X_(2,1) ³ at a location in a2^(nd) row and a 1^(st) column in W₂, X_(2,1) ¹ corresponds to X_(2,1) ²and X_(2,1) ³. When a value of X_(2,1) ¹ is 0, Y_(2,1)=0. A value ofX_(2,1) ² corresponding to X_(2,1) ¹ does not affect a value of Y_(2,1),and a value of X_(2,1) ³ corresponding to X_(2,1) ¹ does not affect thevalue of Y_(2,1), either. Therefore, when the parameter of W₂ indicatedby the PMI2 that is sent by the terminal device to the network deviceincludes all X_(i,l) ¹ in W₂, the terminal device may not feedback, tothe network device, X_(i,l) ² and X_(i,l) ³ that correspond to X_(i,l) ¹whose value is 0. In this way, a quantity of bits required by theterminal device to feed back the PMI2 to the network device can bereduced.

The parameter of W₂ indicated by the PMI2 relates to the N X_(i,l) ¹,whose values are 0, included in W₂ and indicated by the indicationinformation. Assuming that the quantity of all X_(i,l) ¹ whose valuesare 0 included in W₂ is 5, when the indication information indicatesthat W₂ includes five X_(i,l) ¹ whose values are 0, the parameter of W₂corresponding to the PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ² andX_(i,l) ³, which are corresponding to X_(i,l) ¹ whose value is not 0, inW₂ and the parameter of W₂ corresponding to the PMI2 does not includeX_(i,l) ² and X_(i,l) ³, and which are corresponding to X_(i,l) ¹ whosevalues are 0, in W₂. Assuming that the quantity of all X_(i,l) ¹ whosevalues are 0 included in W₂ is 5, when the indication informationindicates that W₂ includes four X_(i,l) ¹ whose values are 0, theindication information does not indicate one X_(i,l) ¹ whose value is 0in W₂, the parameter of W₂ corresponding to the PMI2 includes allX_(i,l) ¹ in W₂, X_(i,l) ² and X_(i,l) ³, which are corresponding toX_(i,l) ¹ whose value is not 0, in W₂, and X_(i,l) ² and X_(i,l) ³,which corresponding to X_(i,l) ¹ whose value is 0 and that is notindicated by the indication information, in W₂, and the parameter of W₂corresponding to the PMI2 does not include X_(i,l) ² and X_(i,l) ³, andwhich are corresponding to the four X_(i,l) ¹ whose value are 0 and thatare indicated by the indication information, in W₂.

Case 2: Y_(i,l) meets the formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³.

Based on Case 2, the indication information included in the CSI is usedto indicate that W₂ includes N X_(i,l) ¹ whose values are 0. This may beunderstood as follows: The indication information is used to indicatethat N X_(i,l) ¹ (whose values are 0) are included in all elements ofW₂. In this case, N represents the quantity of all X_(i,l) ¹ whosevalues are 0 included in W₂. This may be alternatively understood asfollows: The indication information is used to indicate that N X_(i,l) ¹whose values are 0 are included in a part of elements of W₂, and in thiscase, N represents a quantity of a part of all X_(i,l) ¹ whose valuesare 0 included in W₂. For related descriptions about the indicationinformation in Case 2, refer to the related descriptions about theindication information in Case 1. Details are not repeatedly describedherein.

Based on Case 2, the PMI2 included in the CSI is used to indicate aparameter of W₂ in W, and the parameter of W₂ indicated by the PMI2includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding to X_(i,l) ¹other than N X_(i,l) ¹ whose values are 0, in W₂, and the parameter ofW₂ does not include: X_(i,l) ³, corresponding to the N X_(i,l) ¹ whosevalues are 0, in W₂. That X_(i,l) ¹ corresponds to X_(i,l) ³ means that,based on Case 2, X_(i,l) ¹ in Y_(i,l) at any location in W₂ correspondsto X_(i,l) ³ in Y_(i,l). For X_(i,l) ¹ and X_(i,l) ³ that have acorrespondence, a value of X_(i,l) ¹ may determine a value of X_(i,l) ¹.When the value of X_(i,l) ¹ is 0, the value of X_(i,l) ³ correspondingto X_(i,l) ¹ is 0. For example, in an element Y_(2,1)=X_(2,1) ¹×X_(2,1)²×X_(2,1) ³ at a location in a 2^(nd) row and a 1^(st) column in W₂,when a value of X_(2,1) ¹ is 0, Y_(2,1)=0. A value of X_(2,1) ³corresponding to X_(2,1) ¹ does not affect a value of Y_(2,1). Thenetwork device may determine, based on X_(i,l) ¹ whose value is 0, thata value X_(i,l) ³ corresponding to X_(i,l) ¹ is 0. Therefore, when theparameter of W₂ indicated by the PMI2 that is sent by the terminaldevice to the network device includes all X_(i,l) ¹ in W₂, the terminaldevice may not feedback, to the network device, X_(i,l) ³ thatcorresponds to X_(i,l) ¹ whose value is 0. In this way, a quantity ofbits required by the terminal device to feed back the PMI2 to thenetwork device can be reduced.

The parameter of W₂ indicated by the PMI2 relates to the N X_(i,l) ¹,whose values are 0, included in W₂ indicated by the indicationinformation. Assuming that the quantity of all X_(i,l) ¹ whose valuesare 0 included in W₂ is 5, when the indication information indicatesthat W₂ includes five X_(i,l) ¹ whose values are 0, the parameter of W₂corresponding to the PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ³,corresponding to X_(i,l) ¹ whose value is not 0, in W₂, and theparameter of W₂ corresponding to the PMI2 does not include X_(i,l) ³,corresponding to X_(i,l) ¹ whose values are 0, in W₂. Assuming that thequantity of all X_(i,l) ¹ whose values are 0 included in W₂ is 5, whenthe indication information indicates that W₂ includes four X_(i,l) ¹whose values are 0, the indication information does not indicate oneX_(i,l) ¹ whose value is 0 in W₂, the parameter of W₂ corresponding tothe PMI2 includes all X_(i,l) ¹ in W₂, X_(i,l) ³, corresponding toX_(i,l) ¹ whose value is not 0, in W₂, and X_(i,l) ³, corresponding toX_(i,l) ¹ whose value is 0 and that is not indicated by the indicationinformation in W₂, and the parameter of W₂ corresponding to the PMI2does not include X_(i,l) ³, corresponding to the four X_(i,l) ¹ whosevalue are 0 and that are indicated by the indication information, in W₂.

Based on Case 1 or Case 2, a form of the indication information in thisembodiment includes but is not limited to any one of the followingforms.

When that the indication information is used to indicate that W₂includes N X_(i,l) ¹ whose values are 0 is understood as that theindication information is used to indicate that N X_(i,l) ¹ whose valuesare 0 are included in all elements of W₂, and N represents the quantityof all X_(i,l) ¹ whose values are 0 included in W₂, the indicationinformation may be in the following three specific forms.

Form 1: The indication information includes a quantity N of X_(i,l) ¹whose values are 0 in all elements of W₂.

Form 2: The indication information includes a quantity N_(l) of X_(i,l)¹ whose values are 0 in all elements of an l^(th) column vector in W₂,where l is an integer greater than or equal to 0 and less than or equalto L−1, and

${\sum\limits_{l = 0}^{L - 1}N_{l}} = {{N \cdot {\sum\limits_{l = 0}^{L - 1}N_{l}}} = N}$represents that a sum of quantities of X_(i,l) ¹ whose values are 0 inall elements of all column vectors in W₂ is N. For example, when W₂includes one column vector, the indication information includes aquantity N₀ of X_(i,l) ¹ whose values are 0 in all elements of thecolumn vector, and N=N₀. For another example, when W₂ includes twocolumn vectors, i.e., a first column vector and a second column vector,the indication information includes a quantity N₀ of X_(i,l) ¹ whosevalues are 0 in all elements of the first column vector and a quantityN₁ of X_(i,l) ¹ whose values are 0 in all elements of the second columnvector, and N=N₀+N₁.

Form 3: The indication information includes a quantity N_(l) ⁰ ofX_(i,l) ¹ of whose values are 0 in first I elements of an l^(th) columnvector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose values are 0 inlast I elements of the column vector, where l is an integer greater thanor equal to 0 and less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}( {N_{l}^{0} + N_{l}^{1}} )} = {N.}$For example, when W₂ includes one column vector, the indicationinformation includes a quantity N_(l) ⁰ of X_(i,l) ¹ whose values are 0in first I elements of the column vector and a quantity N_(l) ¹ ofX_(i,l) ¹ whose values are 0 in last I elements of the column vector,and N=N₀ ⁰+N₀ ¹. For another example, when W₂ includes two columnvectors, i.e., a first column vector and a second column vector, theindication information includes N₀ ⁰, N₀ ¹, N₁ ⁰, and N₁ ¹, and N=N₀⁰+N₀ ¹+N₁ ⁰+N₁ ¹. N₀ ⁰ represents a quantity of X_(i,l) ¹ whose valuesare 0 in first I elements of the first column vector in W₂, N₀ ¹represents a quantity of X_(i,l) ¹ whose values are 0 in last I elementsof the first column vector in W₂, N₁ ⁰ represents a quantity of X_(i,l)¹ whose values are 0 in first I elements of the second column vector inW₂, and N₁ ¹ and represents a quantity of X_(i,l) ¹ whose values are 0in last I elements of the second column vector in W₂.

When that the indication information is used to indicate that W₂includes N X_(i,l) ¹ whose values are 0 is understood as that theindication information is used to indicate that N X_(i,l) ¹ whose valuesare 0 are included in a part of the elements of W, N represents aquantity of a part of all X_(i,l) ¹ whose values are 0 included in W₂,the indication information may be in the following two specific forms.

Form 4: The indication information includes a quantity N of X_(i,l) ¹whose values are 0 in a part of the elements of W₂.

Form 5: The indication information includes a quantity T_(l) of X_(i,l)¹ whose values are in a part of the elements of an l^(th) column vectorin W₂, where l is an integer greater than or equal to 0 and less than orequal to L−1, and

${\sum\limits_{l = 0}^{L - 1}T_{l}} = {{N \cdot {\sum\limits_{l = 0}^{L - 1}T_{l}}} = N}$represents that a sum of quantities T_(l) of X_(i,l) ¹ whose values are0 in a part of elements of all column vectors in W₂ is N. For example,when W₂ includes one col vector, the indication information includes aquantity T₀ of X_(i,l) ¹ whose values are 0 in a part of elements of thecolumn vector, and T₀=N. For another example, when W₂ includes twocolumn vectors, that is, a first column vector and a second columnvector, the indication information includes a quantity T₀ of X_(i,l) ¹whose values are 0 in a part of elements of the first column vector anda quantity T₁ of X_(i,l) ¹ whose values are 0 in a part of elements ofthe second column vector, and N=T₀+T₁.

It should be noted that the indication information included in the CSIin this embodiment may be used to indicate that W₂ includes N X_(i,l) ¹whose values are 0, or the indication information may be used toindicate that W₂ includes M X_(i,l) ¹ whose values are not 0. Becausethe network device has known a total quantity of X_(i,l) ¹ included inW₂, after receiving the indication information used to indicate that W₂includes M x_(i,l) ¹ whose values are not 0, the network device canobtain, through calculation based on the indication information and thetotal quantity of X_(i,l) ¹ included in W₂, that W₂ includes N X_(i,l) ¹whose values are 0.

S203. The terminal device sends a signal including the CSI to thenetwork device.

Before S203, the terminal device may encode the generated CSI, to obtainthe signal including the CSI. Then, the terminal device sends the signalincluding the CSI to the network device in S203. To ensure that thenetwork device can decode the PMI2 based on the RI and the indicationinformation, when encoding the CSI, the terminal device separatelyencodes the indication information and the PMI2, and also separatelyencodes the RI and the PMI2. In other words, the indication informationand the PMI2 cannot be encoded together in a joint encoding manner, andthe RI and the PMI2 cannot be encoded together in the joint encodingmanner, either. In this way, it can be ensured that the network devicecan determine the PMI2 based on the RI and the indication information.As long as the network device can determine the PMI2 based on the RI andthe indication information, an encoding manner in which the terminaldevice encodes each piece of information included in the CSI is notlimited in this embodiment. In a possible implementation, when encodingthe RI and the indication information that are included in the CSI, theterminal device may encode the RI and the indication information in thejoint encoding manner.

In this embodiment, a first method for jointly encoding the RI and theindication information includes:

-   -   combining, by the terminal device, Q1 bits that carry the RI and        Q2 bits that carry the indication information into Q1+Q2 bits;        and then encoding the Q1+Q2 bits, to obtain D bits after the RI        and the indication information are jointly encoded. For example,        encoding manners, such as repetition coding, Reed-Muller coding,        convolutional coding, or polarization coding can be used to        encode the Q1+Q2 bits, an RI coding manner defined in an LTE        system also may be used to encode the Q1+Q2 bits.

In this embodiment, a second method for jointly encoding the RI and theindication information includes:

-   -   predefining a status value set, where the status value set        includes at least one status value, each of the at least one        status value is used to indicate one type of combination        information of an RI and indication information, the quantity of        bits required to carry the status value is less than a sum of a        quantity of bits required to carry the RI and a quantity of bits        required to carry the indication information, and both the        terminal device and the network device have known the predefined        status value set; and    -   selecting, by the terminal device, a status value from the        predefined status value set, where the selected status value is        used to indicate the combination information, determined by the        terminal device, of the RI and the indication information, and        then encoding the selected status value, to obtain        jointly-encoded information of the RI and the indication        information.

The quantity of bits required to carry the status value is less than thesum of the quantity of bits required to carry the RI and the quantity ofbits required to carry the indication information. Therefore, comparedwith the method for separately carrying the RI and the indicationinformation by using bits, according to the method for jointlyindicating the RI and the indication information by using the statusvalue, a quantity of bits required to indicate the RI and the indicationinformation can be reduced. In this way, resource overheads required bythe terminal device to feed back the CSI to the network device arereduced.

The following describes the predefined status value set by using anexample.

The predefined status value set includes status values 1 to 78, acorrespondence between a status value and combination informationindicated by the status value is as follows:

-   -   the status value 1 indicates: RI=1, and N=0;    -   the status value 2 indicates: RI=1, and N=1;    -   the status value 3 indicates: RI=1, and N=2;    -   the status value 8 indicates: RI=1, and N=7;    -   the status value 9 indicates: RI=2, N₀=0, and N₁=0;    -   the status value 10 indicates: RI=2, N₀=1, and N₁=0;    -   the status value 16 indicates: RI=2, N₀=7, and N₁=0;    -   the status value 17 indicates: RI=2, N₀32 0, and N₁=1;    -   the status value 72 indicates: RI=2, N₀=7, and N₁=7;    -   the status value 73 indicates: RI=3;    -   the status value 74 indicates: RI=4;    -   the status value 75 indicates: RI=5;    -   the status value 76 indicates: RI=6;    -   the status value 77 indicates: RI=7; and    -   the status value 78 indicates: RI=8.

In the foregoing correspondence between a status value and combinationinformation, indicated by the status value, of RI and indicationinformation, N, N₀, or N₁ represents the indication information. For N₀and N, that are indicated by the status values 9 to 72, refer to Form 2in the foregoing related description about the indication information. Ahigh precision codebook is usually applicable to a scenario in which avalue of an RI is relatively small; therefore, in the foregoingcorrespondence, when the RI is 1 or 2, a high precision codebook is usedto feed back a PMI2; when the RI is greater than 2, a low precisioncodebook is used to feedback a PMI2. The indication information relatesonly to the high precision codebook; therefore, in the foregoingcorrespondence, when the RI is greater than 2, there is no indicationinformation corresponding to the status value.

Seven bits are required to carry one of the foregoing 76 status values.When the RI is any one of 1 to 8, 3 bits are required to carry the RI,and at least 6 bits are required to carry the indication information inthe foregoing correspondence. Therefore, compared with the method forseparately carrying the RI and the indication information by using bits,according to the method for jointly indicating the RI and the indicationinformation by using a status value, at least 2 bits can be saved.Assuming that the terminal device determines that RI=1 and N=0 in theCSI, the terminal device selects the status value 1 in the foregoingstatus value set, encodes the status value 1, and sends the encodedstatus value 1 to the network device.

It should be noted that on a premise that neither the RI and the PMI2nor the indication information and the PMI2 are jointly encoded andfurther, the network device can determine the PMI2 based on the RI andthe indication information, a manner in which the terminal device sendsthe CSI to the network device in S202 is not limited in this embodiment.The terminal device may send the CSI to the network device in a periodicreporting manner or in an aperiodic reporting manner. When the terminaldevice sends the CSI to the network device in the periodic reportingmanner, using a schematic diagram of periodically reporting CSI shown inFIG. 3 as an example, the terminal device reports an RI and indicationinformation to the network device at a time point T1, the terminaldevice reports a PMI1 to the network device at a time point T2, and theterminal device reports a PMI2 and a CQI to the network device at a timepoint T3. In this way, the network device can decode the PMI2 based onthe RI and the indication information that are obtained at the timepoint T1 through decoding. When the terminal device sends the CSI to thenetwork device in the aperiodic reporting manner, the terminal deviceneeds to send all information included in the CSI to the network deviceat a same time point.

S204. The network device receives the signal that includes the CSI andthat is sent by the terminal device, and then obtains the RI and theindication information based on the signal including the CSI.

After receiving the signal that includes the CSI and that is sent by theterminal device, the network device decodes bits that are in the signalincluding the CSI and that are used to carry the RI and the indicationinformation, to obtain the RI and the indication information. In apossible implementation, when the signal including the CSI is obtainedby jointly encoding the RI and the indication information by theterminal device, the network device may decode jointly-encodedinformation, to obtain the RI and the indication information.

For the foregoing first method used by the terminal device for jointlyencoding the RI and the indication information, the signal that includesthe CSI and that is received by the network device includes the Dencoded bits. The network device first decodes the D encoded bits, toobtain the Q1+Q2 bits, and then decodes, based on a quantity of theQ1+Q2 bits, a signal that includes the RI and the indication informationand that is in a CSI signal to obtain the RI and the indicationinformation. The RI is represented by using Q1 bits, and the indicationinformation is represented by using Q2 bits, and D=Q1+Q2.

For the foregoing second method used by the terminal device for jointlyencoding the RI and the indication information, the signal that includesthe CSI and that is received by the network device includes an encodedstatus value. The network device first decodes the encoded status value,to obtain a status value, and then determines, based on the status valueand the predefined status value set, the RI and the indicationinformation that are indicated by the status value.

That the network device decodes bits that are in the signal includingthe CSI and that are used to carry the RI and the indicationinformation, to obtain the RI and the indication information includes:

selecting, by the terminal device, a status value that is used toindicate combination information of the RI and the indicationinformation; and

encoding, by the terminal device, the selected status value, to obtainthe signal including the CSI.

S205. The network device obtains, based on the RI and the indicationinformation, the PMI2 included in the CSI.

After obtaining the RI and the indication information, the networkdevice decodes, based on the RI and the indication information, the PMI2included in the encoded CSI, to obtain the PMI2. In this embodiment, thePMI2 may be obtained based on the RI and the indication information in aplurality of manners. In a possible implementation, the network devicefirst determines, based on the RI and the indication information, aquantity of bits required by the PMI2, and then decodes, based on the RIand the quantity of bits required by the PMI2, the PMI2 included in theencoded CSI.

A method for determining, by the network device based on the RI and theindication information, the quantity of bits required by the PMI2includes: determining, based on the RI, a quantity of columns of theprecoding matrix W₂ indicated by the PMI2, and determining, based on theindication information, a quantity of elements that are indicated by thePMI2 and that are in the precoding matrix W₂, to determine the quantityof bits required to carry the PMI2; and determining, by the networkdevice based on the RI and the quantity of bits required by the PMI2, aquantity of bits for decoding the CSI including the PMI2, and decodingthe PMI2 included in the encoded CSI. Alternatively, the network devicedetermines, based on the RI and the indication information, a quantityof bits of the CSI including the PMI2, to decode the CSI including thePMI2.

In this embodiment, the CSI that is sent by the terminal device to thenetwork device may further include a first precoding matrix indicatorPMI1 and/or a CQI. The PMI1 is used to indicate elements in W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1. W₁ is described in detail above, anddetails are not repeatedly described herein. In this embodiment, aparameter included in the PMI1, a parameter included in the CQI, aprocess of sending the PMI1 and the CQI by the terminal device to thenetwork device, and a process of decoding the PMI1 and the CQI by thenetwork device are all similar to those in the prior art, and detailsare not described herein.

S206. The network device determines W based on the RI and the PMI2.

When the CSI that is sent by the terminal device to the network deviceincludes the PMI1, the network device may determine W based on the RI,the PMI1, and the PMI2 in S206. A method includes: determining, by thenetwork device based on the RI, the PMI1, and the PMI2, the precodingmatrix from a predefined precoding matrix group. A specificimplementation method thereof includes: storing, by the network device,all precoding matrices, and determining, based on the RI, the PMI1, andthe PMI2, the precoding matrix from the stored precoding matrices; orgenerating, by the network device, the precoding matrix based on the RI,the PMI1, the PMI2, and a predefined rule.

In the CSI sending and receiving method provided in this embodiment ofthis application, the CSI that is sent by the terminal device to thenetwork device includes the RI, the indication information, and thePMI2. The PMI2 is used to indicate the parameter of W₂, W₂ is a matrixwith 2I rows and L columns, L is the rank indicated by the RI, I is aninteger greater than or equal to 1, Y_(i,l) represents the element atthe location in the i^(th) row and the l^(th) column in W₂, i is aninteger greater than or equal to 0 and less than and equal to 2I−1, andl is an integer greater than or equal to 0 and less than or equal toL−1. Y_(i,l) meets the formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³,X_(i,l) ³ is a complex number with modulus 1, the indication informationis used to indicate that W₂ includes N X_(i,l) ¹ whose values are 0, theparameter of W₂ indicated by the PMI2 includes all X_(i,l) ¹ in W₂ andX_(i,l) ² and X_(i,l) ³, which are corresponding to X_(i,l) ¹ other thanthe N x_(i,l) ¹ whose values are 0, in W₂, and the parameter of W₂ doesnot include X_(i,l) ² and X_(i,l) ³, which are corresponding to the NX_(i,l) ¹ whose values are 0, in W₂. Alternatively, Y_(i,l) meets theformula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is a complex number withmodulus 1, the indication information is used to indicate that W₂includes N X_(i,l) ¹ whose values are 0, the parameter of W₂ includesall X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding to X_(i,l) ¹ other thanthe N X_(i,l) ¹ whose values are 0, in W₂, and the parameter of W₂ doesnot include X_(i,l) ³, corresponding to the N X_(i,l) ¹ whose values are0, in W₂. The network device may obtain the PMI2 by using the RI and theindication information. In the scenario of a high precisioncodebook-based precoding matrix, in the prior art, a PMI2 that is sentby a terminal device to a network device needs to indicate a parameterof all elements of W₂. However, in the technical solutions provided inthis embodiment of this application, the parameter of W₂ indicated bythe PMI2 that is sent by the terminal device to the network device is apart of parameters of elements of W₂. Therefore, the quantity of bitsrequired by the terminal device to send the PMI2 to the network deviceis reduced. The indication information is added to the CSI that is sentby the terminal device to the network device, so that the network devicecan obtain the PMI2 by using the RI and the indication information. Inconclusion, according to the technical solutions provided in thisembodiment of this application, resource overheads required by theterminal device to feed back the CSI to the network device can bereduced in the scenario of a high precision codebook-based precodingmatrix.

Based on the same application concept, an embodiment of this applicationfurther provides a terminal device. The terminal device may implementthe method performed by the terminal device in the method provided inthe embodiment corresponding to FIG. 2. Referring to FIG. 4, theterminal device includes a processing unit 401 and a transceiver unit402.

The processing unit 401 is configured to determine a precoding matrix W;and generate CSI that includes an RI, indication information, and asecond precoding matrix indicator PMI2.

W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L columns,W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a matrix with 2Irows d L columns N_(t) is a quantity of antenna ports, L is a rankindicated by the RI, N_(t) is greater than or equal to L, and I is aninteger greater than or equal to 1; an element at a location in ani^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³is a complex number with modulus 1; and the indication information isused to indicate that W₂ includes N X_(i,l) ¹ whose values are 0. ThePMI2 is used to indicate a parameter of W₂, and the parameter of W₂indicated by the PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ² andX_(i,l) ³, which are corresponding to X_(i,l) ¹ other than the N X_(i,l)¹ whose values are 0, in W₂, and does not include X_(i,l) ² and X_(i,l)³, which are corresponding to the N X_(i,l) ¹ whose values are 0, in W₂.

Alternatively, W meets a formula W=W₁×W₂, W is a matrix with N_(t) rowsand L columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is amatrix with 2I rows and L columns. N_(t) is a quantity of antenna ports,L is a rank indicated by the RI, N_(t) is greater than or equal to L,and I is an integer greater than or equal to 1; an element at a locationin an i^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is acomplex number with modulus 1; and the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0. The PMI2 isused to indicate a parameter of W₂, and the parameter of W₂ indicated bythe PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding toX_(i,l) ¹ other than the N X_(i,l) ¹ whose values are 0, in W₂, and doesnot include X_(i,l) ³, corresponding to the N X_(i,l) ¹ whose values are0, in W₂.

The transceiver unit 402 is configured to send a signal including theCSI to a network device.

In a possible implementation, X_(i,l) ¹ represents a wideband amplitude,X_(i,l) ² represents a subband amplitude, and X_(i,l) ³ represents aphase.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂, where l is an integer greater than or equal to 0 and less        than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}N_{l}} = {N\text{;}}$or

-   -   the indication information includes a quantity N_(l) ⁰ of        X_(i,l) ¹ whose values are 0 in first I elements of an l^(th)        column vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose        values are 0 in last I elements of the column vector, where l is        an integer greater than or equal to 0 and less than or equal to        L−1, and

${\sum\limits_{l = 0}^{L - 1}( {N_{l}^{0} + N_{l}^{1}} )} = {N\text{;}}$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T_(l) of X_(i,l)        ¹ whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where l is an integer greater than or equal to 0        and less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}T_{l}} = {N.}$

In a possible implementation, the processing unit 401 is furtherconfigured to:

-   -   before the transceiver unit 402 sends the signal including the        CSI to the network device, separately encode the indication        information and the PMI2, to obtain the signal including the        CSI.

In a possible implementation, the processing unit 401 is furtherconfigured to:

-   -   before the transceiver unit 402 sends the signal including the        CSI to the network device, encode the RI and the indication        information in a joint encoding manner, to obtain the signal        including the CSI.

In a possible implementation, when encoding the RI and the indicationinformation in the joint encoding manner, to obtain the signal includingthe CSI, the processing unit 401 is specifically configured to:

-   -   represent the RI by using Q1 bits, and represent the indication        information by using Q2 bits;    -   combine the Q1 bits and the Q2 bits into Q1+Q2 bits; and    -   encode the Q1+Q2 bits, to obtain the signal including the CSI.

In a possible implementation, when encoding the RI and the indicationinformation in the joint encoding manner, to obtain the signal includingthe CSI, the processing unit 401 is specifically configured to:

-   -   select a status value that is used to indicate combination        information of the RI and the indication information; and    -   encode the selected status value, to obtain the signal including        the CSI.

In a possible implementation, the CSI further includes:

-   -   a first precoding matrix indicator PMI1, where the PMI is used        to indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, M i S an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1.

It should be noted that unit division in this embodiment of thisapplication is an example, is merely logical function division, and maybe other division in an actual implementation. Functional units in thisembodiment of this application may be integrated into one processingunit, or each of the units ay exist alone physically, or two or moreunits are integrated into one unit. The integrated unit may beimplemented in a form of hardware, or may be implemented in a form of asoftware functional unit.

When the integrated unit is implemented in the form of a softwarefunctional unit and sold or used as an independent product, theintegrated unit may be stored in a computer-readable storage medium.Based on such an understanding, the technical solutions of thisapplication essentially, or the part contributing to the prior art, orall or a part of the technical solutions may be implemented in a form ofa software product. The computer software product is stored in a storagemedium and includes several instructions for instructing a computerdevice (which may be a personal computer, a server, a network device, orthe like) or a processor (processor) to perform all or some of the stepsof the method described in the embodiments of this application. Theforegoing storage medium includes: any medium that can store programcode, such as a Universal Serial Bus (USB) flash drive, a removable harddisk, a read-only memory (ROM), a random access memory (Random AccessMemory, RAM), a magnetic disk, or an optical disc.

Based on the same application concept, an embodiment of this applicationfurther provides a terminal device. The terminal device uses the methodperformed by the terminal device in the method provided in theembodiment corresponding to FIG. 2, and may be a device that is the sameas the terminal device shown in FIG. 4. Referring to FIG. 5, theterminal device includes a processor 501, a transceiver 502, and amemory 503.

The processor 501 is configured to read a program in the memory 503, toexecute the following process:

-   -   determining a precoding matrix W; and generating CSI that        includes an RI, indication information, and a second precoding        matrix indicator PMI2.

W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L columns,W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a matrix with 2Irows and L columns. N_(t) is a quantity of antenna ports, L is a rankindicated by the RI, N_(t) is greater than or equal to L, and I is aninteger greater than or equal to 1; an element at a location in ani^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³is a complex number with modulus 1; and the indication information isused to indicate that W₂ includes N X_(i,l) ¹ whose values are 0. ThePMI2 is used to indicate a parameter of W₂, and the parameter of W₂indicated by the PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ² andX_(i,l) ³, and which are corresponding to X_(i,l) ¹ other than the NX_(i,l) ¹, whose values are 0, in W₂, and does not include X_(i,l) ² andX_(i,l) ³, which are corresponding to the N X_(i,l) ¹ whose values are0, in W₂.

Alternatively, W meets a formula W=W₁×W₂, W is a matrix with N_(t) rowsand L columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is amatrix with 2I rows and L columns, N_(t) is a quantity of antenna ports,L is a rank indicated by the RI, N_(t) is greater than or equal to L,and I is an integer greater than or equal to 1; an element at a locationin an i^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andmeets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is a complexnumber with modulus 1; and the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0. The PMI2 isused to indicate a parameter of W₂, and the parameter of W₂ indicated bythe PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding toX_(i,l) ¹ other than the N X_(i,l) ¹ whose values are 0, in W₂, and doesnot include X_(i,l) ³, corresponding to the N X_(i,l) ¹ whose values are0, in W₂.

The processor 501 is further configured to send, by using thetransceiver 502, the CSI to a network device.

The transceiver 502 is configured to receive and send data under controlof the processor 501.

In a possible implementation, X_(i,l) ¹ represents a wideband amplitude,X_(i,l) ² represents a subband amplitude, and X_(i,l) ³ represents aphase.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂, where l is an integer greater than or equal to 0 and less        than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}N_{l}} = {N\text{;}}$or

-   -   the indication information includes a quantity N_(l) ⁰ of        N_(i,l) ¹ whose values are 0 in first I elements of an l^(th)        column vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose        values are 0 in last I elements of the column vector, where l is        an integer greater than or equal to 0 and less than or equal to        L−1, and

${\sum\limits_{l = 0}^{L - 1}( {N_{l}^{0} + N_{l}^{1}} )} = {N\text{;}}$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T_(l) of X_(i,l)        ¹ whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where is an integer greater than or equal to 0 and        less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}T_{l}} = {N.}$

In a possible implementation, the processor 501 is further configuredto:

-   -   before the transceiver 502 sends a signal including the CSI to        the network device, separately encode the indication information        and the PMI2, to obtain the signal including the CSI.

In a possible implementation, the processor 501 is further configuredto:

-   -   before the transceiver 502 sends the signal including the CSI to        the network device, encode the RI and the indication information        in a joint encoding manner, to obtain the signal including the        CSI.

In a possible implementation, when encoding the RI and the indicationinformation in the joint encoding manner, to obtain the signal includingthe CSI, the processor 501 is specifically configured to:

-   -   represent the RI by using Q1 bits, and represent the indication        information by using bits;    -   combine the Q1 bits and the Q2 bits into Q1+Q2 bits; and    -   encode the Q1+Q2 bits, to obtain the signal including the CSI.

In a possible implementation, when encoding the RI and the indicationinformation in the joint encoding manner, to obtain the signal includingthe CSI, the processor 501 is specifically configured to:

-   -   select a status value that is used to indicate combination        information of the RI and the indication information: and    -   encode the selected status value, to obtain the signal including        the CSI.

In a possible implementation, the CSI further includes:

-   -   a first precoding matrix indicator PMI1, where the PMI is used        to indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1.

The processor 501, the transceiver 502, and the memory 503 are connectedto each other by using a bus. The bus may be a peripheral componentinterconnect (PCI) bus, an extended industry standard architecture(EISA) bus, or the like. The bus may be classified into an address bus,a data bus, a control bus, and the like.

In FIG. 5, a bus architecture may include any quantity of interconnectedbuses and bridges, and specifically connects together a circuit of oneor more processors represented by the processor 501 and a circuit of amemory represented by the memory 503. The bus architecture may furtherconnect together various other circuits such as a peripheral device, avoltage stabilizer, and a power management circuit. These are well knownin the art, and therefore are not further described in thisspecification. A bus interface provides an interface. The transceiver502 may be a plurality of components. To be specific, the transceiver502 includes a transmitter and a receiver and provides units configuredto communicate with various other apparatuses on a transmission medium.The processor 501 is responsible for management of the bus architectureand general processing, and the memory 503 may store data that is usedwhen the processor 501 performs an operation.

Optionally, the processor 501 may be a central processing unit, anapplication-specific integrated circuit (ASIC), a field programmablegate array (FPGA), or a complex programmable logic device (CPLD).

An embodiment of this application further provides a computer storagemedium. The storage medium stores a software program. When being readand executed by one or more processors, the software program is capableof implementing the CSI sending method performed by the terminal devicein the foregoing embodiment.

An embodiment of this application further provides a terminal device,including at least one chip configured to perform the CSI sending methodperformed by the terminal device in the foregoing embodiment.

An embodiment of this application provides a computer program productincluding an instruction. When running on a computer, the computerprogram product enables the computer to perform the CSI sending methodperformed by the terminal device in the foregoing embodiment.

Based on the same application concept, an embodiment of this applicationfurther provides a network device. The network device may implement themethod performed by the network device in the method provided in theembodiment corresponding to FIG. 2. Referring to FIG. 6, the networkdevice includes a transceiver unit 601 and a processing unit 602.

The transceiver unit 601 is configured to receive CSI that is sent by aterminal device, where the CSI includes an RI, indication information,and a second precoding matrix indicator PMI2.

The processing unit 602 is configured to: obtain the RI and theindication information that are included in the CSI; obtain, based onthe RI and the indication information, the PMI2 included in the CSI; anddetermine a precoding matrix W based on the RI and the second precodingmatrix indicator PMI2.

W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L columns,W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a matrix with 2Irows and L columns. N_(t) is a quantity of antenna ports, L is a rankindicated by the RI, N_(t) is greater than or equal to L, and I is aninteger greater than or equal to 1; an element at a location in ani^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³is a complex number with modulus 1; and the indication information isused to indicate that W₂ includes N X_(i,l) ¹ whose values are 0, thePMI2 is used to indicate a parameter of W₂, and the parameter of W₂indicated by the PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ² andX_(i,l) ³, which are corresponding to X_(i,l) ¹ other than the N X_(i,l)¹ whose values are 0, in W₂, and does not include X_(i,l) ² and X_(i,l)³, which are corresponding to the N X_(i,l) ¹ whose values are 0, in W₂.

Alternatively, W meets a formula W=W₁×W₂, W is a matrix N_(t) rows and Lcolumns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a matrixwith 2I rows and L columns, N_(t) is a quantity of antenna ports, L is arank indicated by the RI, N_(t) is greater than or equal to L, and 1 isan integer greater than or equal to 1; an element at a location in ani^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, I is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is acomplex number with modulus 1; and the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0. The PMI2 isused to indicate a parameter of W₂, and the parameter of W₂ indicated bythe PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding toX_(i,l) ¹ other than N X_(i,l) ¹ whose values are 0, in W₂, and does notinclude X_(i,l) ³, corresponding to the N X_(i,l) ¹ whose values are 0,in W₂.

In a possible implementation, X_(i,l) ¹ represents a wideband amplitude,X_(i,l) ² represents a subband amplitude, and X_(i,l) ³ represents aphase.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂, where 1 is an integer greater than or equal to 0 and less        than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}N_{l}} = {N\text{;}}$or

-   -   the indication information includes a quantity N_(l) ⁰ of        X_(i,l) ¹ whose values are 0 in first I elements of an l^(th)        column vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose        values are 0 in last I elements of the column vector, where 1 is        an integer greater than or equal to 0 and less than or equal to        L−1, and

${\sum\limits_{l = 0}^{L - 1}( {N_{l}^{0} + N_{l}^{1}} )} = {N\text{;}}$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T_(l) of X_(i,l)        ¹ whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where 1 is an integer greater than or equal to 0        and less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}T_{l}} = {N.}$

In a possible implementation, when obtaining the RI and the indicationinformation based on a signal including the CSI, the processing unit 602is specifically configured to:

-   -   decode bits that are in the signal including the CSI and that        are used to carry the RI and the indication information, to        obtain the RI and the indication information.

When obtaining the PMI2 based on the RI and the indication information,the processing unit 602 is specifically configured to:

-   -   decode, based on the RI and the indication information, a bit        that is in the signal including the CSI and that is used to        carry the PMI2, to obtain the PMI2.

In a possible implementation, when decoding, based on the RI and theindication information, the bit that is in the signal including the CSIand that is used to carry the PMI2, to obtain the PMI2, the processingunit 602 is specifically configured to:

-   -   determine, based on the RI and the indication information, a        quantity of bits required to decode the PMI2; and    -   decode, based on the RI and the quantity of bits, the bit that        is used to carry the PMI2, to obtain the PMI2.

In a possible implementation, when decoding the bits that include the RIand the indication information and that are in the CSI signal, to obtainthe RI and the indication information, the processing unit 602 isspecifically configured to:

-   -   decode, based on a quantity Q1+Q2 of hits, a signal that        includes the RI and the indication information and that is in        the CSI signal, to obtain the RI and the indication information.

The RI is represented by using Q1 bits, and the indication informationis represented by using Q2 bits.

When decoding the bits that are in the signal including the CSI and thatare used to carry the RI and the indication information, to obtain theRI and the indication information, the processing unit 602 isspecifically configured to:

-   -   obtain a status value based on the bits that are used to carry        the RI and the indication information, where the status value is        used to indicate combination information of the RI and the        indication information; and    -   obtain the RI and the indication information based on the status        value.

In a possible implementation, the CSI further includes:

-   -   a first precoding matrix indicator PMI, where the PMI is used to        indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1.

When determining W based on the RI and the PMI2, the processing unit 602is specifically configured to:

-   -   determine W based on the RI, the PMI1, and the PMI2.

Based on the same application concept, an embodiment of this applicationfurther provides a network device. The network device uses the methodperformed by the network device in the method provided in the embodimentcorresponding to FIG. 2, and may be a device that is the same as thenetwork device shown in FIG. 6. Referring to FIG. 7, the network deviceincludes a processor 701, a transceiver 702, and a memory 703.

The processor 701 is configured to read a program in the memory 703, toexecute the following process:

-   -   receiving, by using the transceiver 702, CSI that is sent by a        terminal device, where the CSI includes an RI, indication        information, and a second precoding matrix indicator PMI2.

The processor 701 is further configured to: obtain the RI and theindication information that are included in the CSI; obtain, based onthe RI and the indication information, the PMI2 included in the CSI; anddetermine a precoding matrix W based on the RI and the second precodingmatrix indicator PMI2.

W meets a formula W=W₁×W₂, W is a matrix with N_(t) rows and L columns,W₁ is a matrix with N_(t) rows and 2I columns, W₂ is a matrix with 2Irows and L columns. N_(t) is a quantity of antenna ports, L is a rankindicated by the RI, N_(t) is greater than or equal to L, and I is aninteger greater than or equal to 1; an element at a location in ani^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ²×X_(i,l) ³, X_(i,l) ³is a complex number with modulus 1; and the indication information isused to indicate that W₂ includes N X_(i,l) ¹ whose values are 0. ThePMI2 is used to indicate a parameter of W₂, and the parameter of W₂indicated by the PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ² andX_(i,l) ³, which are corresponding to X_(i,l) ¹ other than the N X_(i,l)¹ whose values are 0, in W₂, and does not include X_(i,l) ² and X_(i,l)³, which are corresponding to the N X_(i,l) ¹ whose values are 0, in W₂.

Alternatively, W meets a formula W=W₁×W₂, W is a matrix with N_(t) rowsand L columns, W₁ is a matrix with N_(t) rows and 2I columns, W₂ is amatrix with 2I rows and L columns. N_(t) is a quantity of antenna ports,L is a rank indicated by the RI, N_(t) is greater than or equal to L,and I is an integer greater than or equal to 1; an element at a locationin an i^(th) row and an l^(th) column in W₂ is Y_(i,l), i is an integergreater than or equal to 0 and less than or equal to 2I−1, l is aninteger greater than or equal to 0 and less than or equal to L−1, andY_(i,l) meets a formula Y_(i,l)=X_(i,l) ¹×X_(i,l) ³, X_(i,l) ³ is acomplex number with modulus 1; and the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0, the PMI2 isused to indicate a parameter of W₂, and the parameter of W₂ indicated bythe PMI2 includes all X_(i,l) ¹ in W₂ and X_(i,l) ³, corresponding toX_(i,l) ¹ other than the N X_(i,l) ¹ whose values are 0, in W₁ and doesnot include X_(i,l) ¹, corresponding to the N X_(i,l) ¹ whose values are0, in W₂.

The transceiver 702 is configured to receive and send data under controlof the processor 701.

In a possible implementation, X_(i,l) ¹ represents a wideband amplitude,X_(i,l) ² represents a subband amplitude, and X_(i,l) ³ represents aphase.

In a possible implementation, that the indication information is used toindicate that W₂ includes N X_(i,l) ¹ whose values are 0 isspecifically:

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in all elements of W₂; or    -   the indication information includes a quantity N_(l) of X_(i,l)        ¹ whose values are 0 in all elements of an l^(th) column vector        in W₂, where l is an integer greater than or equal to 0 and less        than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}N_{l}} = {N\text{;}}$or

-   -   the indication information includes a quantity N_(l) ⁰ of        X_(i,l) ¹ whose values are 0 in first I elements of an l^(th)        column vector in W₂ and a quantity N_(l) ¹ of X_(i,l) ¹ whose        values are 0 in last I elements of the column vector, where l is        an integer greater than or equal to 0 and less than or equal to        L−1, and

${\sum\limits_{l = 0}^{L - 1}( {N_{l}^{0} + N_{l}^{1}} )} = {N\text{;}}$or

-   -   the indication information includes a quantity N of X_(i,l) ¹        whose values are 0 in a part of elements of W₂; or    -   the indication information includes a quantity T₁ of X_(i,l) ¹        whose values are 0 in a part of elements of an l^(th) column        vector in W₂, where l is an integer greater than or equal to 0        and less than or equal to L−1, and

${\sum\limits_{l = 0}^{L - 1}T_{l}} = {N.}$

In a possible implementation, when obtaining the RI and the indicationinformation based on a signal including the CSI, the processor 701 isspecifically configured to:

-   -   decode bits that are in the signal including the CSI and that        are used to carry the RI and the indication information, to        obtain the RI and the indication information.

When obtaining the PMI2 based on the RI and the indication information,the processor 701 is specifically configured to:

-   -   decode, based on the RI and the indication information, a bit        that is in the signal including the CSI and that is used to        carry the PMI2, to obtain the PMI2.

In a possible implementation, when decoding, based on the RI and theindication information, the bit that is in the signal including the CSIand that is used to carry the PMI2, to obtain the PMI2, the processor701 is specifically configured to:

-   -   determine, based on the RI and the indication information, the        quantity of bits required to decode the PMI2; and    -   decode, based on the RI and the quantity of bits, the bit that        is used to carry the PMI2, to obtain the PMI2.

In a possible implementation, when decoding the bits that include the RIand the indication information and that are in the CSI signal, to obtainthe RI and the indication information, the processor 701 is specificallyconfigured to:

-   -   decode, based on a quantity Q1+Q2 of bits, a signal that        includes the RI and the indication information and that is in        the CSI signal, to obtain the RI and the indication information.

The RI is represented by using Q1 bits, and the indication informationis represented by using Q2 bits.

In a possible implementation, when decoding the bits that are in thesignal including the CSI and that are used to carry the RI and theindication information, to obtain the RI and the indication information,the processor 701 is specifically configured to:

-   -   obtain a status value based on the bits that are used to carry        the RI and the indication information, where the status value is        used to indicate combination information of the RI and the        indication information; and    -   obtain the RI and the indication information based on the status        value.

In a possible implementation, the CSI further includes:

-   -   a first precoding matrix indicator PMI1, where the PMI is used        to indicate W₁, W₁ meets

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$X₁ is a matrix with N_(t)/2 rows and I columns, X₁=[v₀ . . . v_(M-1)],v_(m) is a column vector including N_(t)/2 elements, to i S an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to 1.

When determining W based on the RI and the PMI2, the processor 701 isspecifically configured to:

-   -   determine W based on the RI, the PMI1, and the PMI2.

The processor 701, the transceiver 702, and the memory 703 are connectedto each other by using a bus. The bus may be a PCI bus, an EISA bus, orthe like. The bus may be classified into an address bus, a data bus, acontrol bus, and the like.

In FIG. 7, a bus architecture may include any quantity of interconnectedbuses and bridges, and specifically connects together a circuit of oneor more processors represented by the processor 701 and a circuit of amemory represented by the memory 703. The bus architecture may furtherconnect together various other circuits such as a peripheral device, avoltage stabilizer, and a power management circuit. These are well knownin the art, and therefore are not further described in thisspecification. A bus interface provides an interface. The transceiver702 may be a plurality of components. To be specific, the transceiver702 includes a transmitter and a receiver and provides units configuredto communicate with various other apparatuses on a transmission medium.The processor 701 is responsible for management of the bus architectureand general processing, and the memory 703 may store data that is usedwhen the processor 701 performs an operation.

Optionally, the processor 701 may be a central processing unit, an ASIC,an FPGA, or a CPLD.

An embodiment of this application further provides a computer storagemedium. The storage medium stores a software program. When being readand executed by one or more processors, the software program is capableof implementing the CSI receiving method performed by the network devicein the foregoing embodiment.

An embodiment of this application further provides a network device,including at least one chip configured to perform the CSI receivingmethod performed by the network device in the foregoing embodiment.

An embodiment of this application provides a computer program productincluding an instruction. When running on a computer, the computerprogram product enables the computer to perform the CSI receiving methodperformed by the network device in the foregoing embodiment.

Based on the same concept, an embodiment of this application furtherprovides a communications system. As shown in FIG. 8, the communicationssystem includes a terminal device 801 and a network device 802. Theterminal device 801 is configured to perform the method performed by theterminal device in the method provided in the embodiment correspondingto FIG. 2, and the terminal device 801 may be a device that is the sameas the terminal device shown in FIG. 4 or FIG. 5. The network device 802is configured to perform the method performed by the network device inthe method provided in the embodiment corresponding to FIG. 2, and thenetwork device 802 may be a device that is the same as the networkdevice shown in FIG. 6 or FIG. 7. The communications system can be usedto implement the CSI sending and receiving method provided in theembodiments of this application.

A person skilled in the art should understand that the embodiments ofthis application may be provided as a method, a system, or a computerprogram product. Therefore, this application may use a form of hardwareonly embodiments, software only embodiments, or embodiments with acombination of software and hardware. Moreover, this application may usea form of a computer program product that is implemented on one or morecomputer-usable storage media (including but not limited to a diskmemory, a CD-ROM, an optical memory, and the like) that include computerusable program code.

This application is described with reference to the flowcharts and/orblock diagrams of the method, the device (system), and the computerprogram product according to this application. It should be understoodthat computer program instructions may be used to implement each processand/or each block in the flowcharts and/or the block diagrams and acombination of a process and/or a block in the flowcharts and/or theblock diagrams. These computer program instructions may be provided fora general-purpose computer, a dedicated computer, an embedded processor,or a processor of any other programmable data processing device togenerate a machine, so that the instructions executed by a computer or aprocessor of any other programmable data processing device generate anapparatus for implementing a specified function in one or more processesin the flowcharts and/or in one or more blocks in the block diagrams.

These computer program instructions may be stored in a computer readablememory that can instruct the computer or any other programmable dataprocessing device to work in a specific manner, so that the instructionsstored in the computer readable memory generate an artifact thatincludes an instruction apparatus. The instruction apparatus implementsa specified function in one or more processes in the flowcharts and/orin one or more blocks in the block diagrams.

These computer program instructions may be loaded onto a computer oranother programmable data processing device, so that a series ofoperations and steps are performed on the computer or the anotherprogrammable device, thereby generating computer-implemented processing.Therefore, the instructions executed on the computer or the anotherprogrammable device provide steps for implementing a specified functionin one or more processes in the flowcharts and/or in one or more blocksin the block diagrams.

Obviously, a person skilled in the art can make various modificationsand variations to this application without departing from the spirit andscope of this application. This application is intended to cover thesemodifications and variations of this application provided that they fallwithin the scope of protection defined by the following claims of thisapplication and their equivalent technologies.

What is claimed is:
 1. A method for wireless communications for anetwork device, comprising: receiving, from a terminal device, channelstate information (CSI) comprising a rank indicator (RI), indicationinformation, and a second precoding matrix indicator (PMI2); anddetermining a precoding matrix W based on the CSI, wherein W satisfiesW=W₁×W₂, W is an N_(t) by L matrix, W₁ is an N_(t) by 2I matrix, W₂ is a2I by L matrix, L is a rank indicated by the RI, N_(t) is greater thanor equal to L, and I is an integer greater than or equal to 1, whereinan element at a location in an i^(th) row and an l^(th) column in W₂ isrepresented as Y_(i,l), wherein i is an integer greater than or equal to0 and less than or equal to 2I-1, l is an integer greater than or equalto 0 and less than or equal to L-1, Y_(i,l) satisfies Y_(i,l)=X¹_(i,l)×X³ _(i,l), where X¹ _(i,l) represents an amplitude of a widebandchannel, X³ _(i,l) is a complex number with modulus 1 and represents aphase; and the indication information indicates that M X¹ _(i,l) in W₂have nonzero values, the PMI2 indicates a parameter of W₂, the parametercomprises all X¹ _(i,l) in W₂ and X³ _(i,l) corresponding to the M X¹_(i,l) in W₂, and the parameter does not comprise X³ _(i,l)corresponding to X¹ _(i,l) other than the M X¹ _(i,l).
 2. The methodaccording to claim 1, wherein the indication information indicates aquantity M_(l) of X_(i,l) ¹ whose values are not 0 in all elements of anl^(th) column vector in W₂, wherein l is an integer greater than orequal to 0 and less than or equal to L−1, and${\sum\limits_{l = 0}^{L - 1}M_{l}} = {M.}$
 3. The method according toclaim 1, further comprising: determining, based on the RI and theindication information, a quantity of bits comprised in the PMI2; andobtaining the PMI2.
 4. The method according to claim 1, wherein the CSIfurther comprises: a first precoding matrix indicator(PMI1), wherein thefirst PMI1 indicates W₁, where ${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$ X₁ is a N_(t)/2 by I matrix, X₁=[v₀ . . . v_(I-1)],v_(m) is a column vector comprising N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to 1, and I is aninteger greater than or equal to
 1. 5. The method according to claim 4,wherein the precoding matrix W is further determined based on a firstprecoding matrix (PMI1).
 6. An apparatus, comprising: a transceiver; atleast one processor; and a non-transitory computer-readable storagemedium coupled to the at least one processor and storing programminginstructions for execution by the at least one processor, theprogramming instructions instruct the at least one processor to: causethe transceiver to receive, from a terminal device, channel stateinformation (CSI) comprising a rank indicator (RI), indicationinformation, and a second precoding matrix indicator (PMI2); anddetermine a precoding matrix W based on the CSI, wherein W satisfiesW=W₁×W₂, W is an N_(t) by L matrix, W₁ is an N_(t) by 2I matrix, W₂ is a2I by L matrix, L is a rank indicated by the RI, N_(t) is greater thanor equal to L, and I is an integer greater than or equal to 1, whereinan element at a location in an i^(th) row and an l^(th) column in W₂ isrepresented as Y_(i,l), wherein i is an integer greater than or equal to0 and less than or equal to 2I-1, l is an integer greater than or equalto 0 and less than or equal to L-1, Y_(i,l) satisfies Y_(i,l)=X¹_(i,l)×X³ _(i,l), wherein X¹ _(i,l) represents an amplitude of awideband channel, X³ _(i,l) is a complex number with modulus 1 andrepresents a phase; and the indication information indicates that M X¹_(i,l) in W₂ have nonzero values, the second PMI indicates a parameterof W₂, the parameter comprises all X¹ _(i,l) in W₂ and X³ _(i,l)corresponding to the M X¹ _(i,l) in W₂, and the parameter does notcomprise X³ _(i,l) corresponding to X¹ _(i,l) other than the M X¹_(i,l).
 7. The apparatus according to claim 6, wherein the indicationinformation indicates a quantity M_(l) of X_(i,l) ¹ whose values are not0 in all elements of an l^(th) column vector in W₂, and${\sum\limits_{l = 0}^{L - 1}M_{l}} = {M.}$
 8. The apparatus accordingto claim 6, wherein the programming instructions further instruct the atleast one processor to: determine, based on the RI and the indicationinformation, a quantity of bits required to decode the PMI2; and obtainthe PMI2.
 9. The apparatus according to claim 6, wherein the CSI furthercomprises: a first precoding matrix indicator (PMI1), wherein the PMI1is used to indicate W₁, where ${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$ X_(l) is a N_(t)/2 by I matrix, X₁=[v₀ . . . v_(I-1)],v_(m) is a column vector comprising N_(t)/2 elements, m is an integergreater than or equal to 0 and less than or equal to I−1, and I is aninteger greater than or equal to
 1. 10. The apparatus according to claim9, wherein the precoding matrix W is further determined based on PMI1.11. A non-transitory computer readable storage medium, comprisingcomputer program codes executable by at least one processor and cause anapparatus to perform the steps of: receiving from a terminal device,channel state information (CSI) comprising a rank indicator (RI),indication information, and a second precoding matrix indicator (PMI2);and determining a precoding matrix W based on the CSI, wherein Wsatisfies W=W₁×W₂, W is an N_(t) L matrix, W₁ is an N_(t) by 2I matrix,W₂ is a 2I by L matrix, L is a rank indicated by the RI, N_(t) isgreater than or equal to L, and I is an integer greater than or equal to1, wherein an element at a location in an i^(th) row and an l^(th)column in W₂ is represented as Y_(i,l), wherein i is an integer greaterthan or equal to 0 and less than or equal to 2I-1, l is an integergreater than or equal to 0 and less than or equal to L-1, Y_(i,l)satisfies Y_(i,l)=X¹ _(i,l)×X³ _(i,l), where X¹ _(i,l) represents anamplitude of a wideband channel, X³ _(i,l) is a complex number withmodulus 1 and represents a phase; and the indication informationindicates that M X¹ _(i,l) in W₂ have nonzero values, the PMI2 indicatesa parameter of W₂, the parameter comprises all X¹ _(i,l) in W₂ and X³_(i,l) corresponding to the M X¹ _(i,l) in W₂, and the parameter doesnot comprise X³ _(i,l) corresponding to X¹ _(i,l) other than the M X¹_(i,l).
 12. The non-transitory computer readable storage mediumaccording to claim 11, wherein the indication information indicates aquantity M_(l) of X¹ _(i,l) whose values are not 0 in all elements of anl^(th) column vector in W₂, and${{\sum\limits_{l = 0}^{L - 1}M_{l}} = M}.$
 13. A channel stateinformation CSI receiving method for a network device, comprising:receiving, from a terminal device, channel state information (CSI)comprising a rank indicator (RI), indication information, and a secondprecoding matrix indicator (PMI2); and determining a precoding matrix Wbased on the CSI, W satisfies W=W₁×W₂, W is an N_(t) L matrix, W₁ is anN_(t) by 2I matrix, W₂ is a 2I by L matrix, L is a rank indicated by theRI, N_(t)≥L, I≥1, wherein an element at a location in an i^(th) row andan l^(th) column in W₂ is represented as Y_(i,l), 2I-1≥i≥0, L-1≥l≥0,Y_(i,l) satisfies Y_(i,l)=X¹ _(i,l)×X² _(i,l)×X³ _(i,l)represents awideband amplitude, X² _(i,l) represents a subband amplitude, and X₃_(i,l) is a complex number with modulus 1 and represents a phase; andthe indication information indicates that M X¹ _(i,l) in W₂ have nonzerovalues, the PMI2 indicates a parameter of W₂, the parameter comprisesall X₁ _(i,l) in W₂ and X² _(i,l) and X³ _(i,l), which correspond to theM X¹ _(i,l) in W₂, and the parameter does not comprise X² _(i,l) and X³_(i,l) which correspond to X¹ _(i,l) other than the M X¹ _(i,l).
 14. Themethod according to claim 13, wherein the indication informationindicates a quantity M_(l) of X¹ _(i,l) whose values are not 0 in allelements of an l^(th) column vector in W₂, and${{\sum\limits_{l = 0}^{L - 1}M_{l}} = M}.$
 15. The method according toclaim 13, further comprising: determining, based on the RI and theindication information, a quantity of bits comprised in the PMI2; andobtaining the PMI2.
 16. The method according to claim 13, wherein theCSI further comprises: a first precoding matrix indicator (PMI1),wherein the PMI1 indicates W₁, where ${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$ X₁ is a N_(t)/2 by I matrix, X₁=[v₀ . . . v₁₋₂], v_(m)is a column vector comprising N_(t)/2 elements, m is an integer greaterthan or equal to 0 and less than or equal to I-1.
 17. The methodaccording to claim 16, wherein the precoding matrix W is furtherdetermined based on the PMI1.
 18. An apparatus, comprising: atransceiver; at least one processor; and a non-transitorycomputer-readable storage medium coupled to the at least one processorand storing programming instructions for execution by the at least oneprocessor, the programming instructions instruct the at least oneprocessor to: cause the transceiver to receive, from a terminal device,channel state information (CSI) comprising a rank indicator (RI),indication information, and a second precoding matrix indicator (PMI2);and determine a precoding matrix W based on the CSI, wherein W satisfiesW=W₁×W₂, W is an N_(t) by L matrix , W₁ is an N_(t) by 2I matrix, W₂ isa 2I by L matrix, L is a rank indicated by the RI, N_(t≥L, I≥)1, whereinan element at a location in an i^(th) row and an l^(th) column in W₂ isrepresented as Y_(i,l), 2I-1≥i≥0, L-1≥l≥0, Y_(i,l) satisfies Y_(i,l)=X¹_(i,l)×X² _(i,l)×X³ _(i,l), and X¹ _(i,l) represents a widebandamplitude, X² _(i,l) represents a subband amplitude, and X³ _(i,l) is acomplex number with modulus 1 and represents a phase; and the indicationinformation indicates that M X¹ _(i,l) in W₂ have nonzero values, thePMI2 indicates a parameter of W₂, the parameter comprises all X¹ _(i,l)in W₂ and X² _(i,l) and X³ _(i,l), which correspond to the M X¹ _(i,l)in W₂, and the parameter does not comprise X² _(i,l) and X³ _(i,l) whichcorrespond to X¹ _(i,l) other than the M X¹ _(i,l).
 19. The apparatusaccording to claim 18, wherein the indication information indicates aquantity M₁ of X¹ _(i,l) whose values are not 0 in all elements of anl^(th) column vector in W₂, and${{\sum\limits_{l = 0}^{L - 1}M_{l}} = M}.$
 20. The apparatus accordingto claim 18, wherein the programming instructions further instruct theat least one processor to: determine, based on the RI and the indicationinformation, a quantity of bits required to decode the PMI2; and obtainthe PMI2.
 21. The apparatus according to claim 18, wherein the CSIfurther comprises: a first precoding matrix indicator (PMI1), whereinthe PMI1 is used to indicate W₁, where ${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$ X, is a N_(t)/2 by I matrix, X₁=[v₀ . . . v_(l-1)],v_(m) is a column vector comprising N_(t)/2 elements,and m is an integergreater than or equal to 0 and less than or equal to I-1.
 22. Theapparatus according to claim 18, wherein the programming instructionsfurther instruct the at least one processor to: the precoding matrix Wis further determined based on PMI1.
 23. A non-transitory computerreadable storage medium, comprising computer program codes executable byat least one processor and cause an apparatus to perform the steps of:receiving channel state information (CSI) comprising a rank indicator(RI), indication information, and a second precoding matrix indicator(PMI2); and determining a precoding matrix W based on the CSI, wherein Wsatisfies W=W₁×W₂, W is an N_(t) by L matrix, W₁ is an N_(t) by 2Imatrix, W₂ is a 2I by L matrix, L is a rank indicated by the RI,N_(t)≥L, I≥1, wherein an element at a location in an i^(th) row and anl^(th) column in W₂ is represented as Y_(i,l), 2I-1≥i≥0, L-1≥l≥0,Y_(i,l) satisfies Y_(i,l)=X¹ _(i,l)×X² _(i,l)×X³ _(i,l), and X¹ _(i,l)represents a wideband amplitude, X² _(i,l) represents a subbandamplitude, and X³ _(i,l) is a complex number with modulus 1 andrepresents a phase; and the indication information indicates that M X¹_(i,l) in W₂ have nonzero values, the PMI2 indicates a parameter of W₂,the parameter comprises all X¹ _(i,l) in W₂ and X² _(i,l) and X³ _(i,l),which correspond to the M X¹ _(i,l) in W₂, and the parameter does notcomprise X² _(i,l) and X³ _(i,l) which correspond to X¹ _(i,l) otherthan the M X¹ _(i,l).
 24. The non-transitory computer readable storagemedium according to claim 23, wherein the indication informationindicates a quantity M_(l) of X¹ _(i,l) whose values are not 0 in allelements of an l^(th) column vector in W₂ , and${{\sum\limits_{l = 0}^{L - 1}M_{l}} = M}.$